I’ve contacted support about this, but I’ve got the standard response back about factory reset etc. I linked to this thread in my ticket to show it’s not an issue with just my phone but an issue with how the fingerprint sensor is configured out of the box.
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Well that may not be the case. Sure if every user had the same problem ;but this forum only represents probably much less than 1% of users and even they may not all subscribe to this problem.
If it’s a proven “solution” then sure, it may help a bit, but this issue is quite old and yet unresolved. I don’t have this issue though. One person here mentioned switching to a custom ROM fixed it. Which may mean a factory reset works, or some hobby devs do a better job than FP. In either case, FP should step up with they’re QA. If you really want to see it solved, try and get through the FP support auto response layer until you talk to someone who knows more and can do more for you.
I doubt a reset will change anything, mine got worse over time, in the meantime I have it without case as well…
My ticket is somehow “open”…
It’s still the same on CalyxOS, as sensitive as ever, so it probably hasn’t been fixed in any LOS (adjacent) ROM.
I still think this is a driver / firmware issue and needs to get fixed somewhere in the proprietary blobs.
It might be fixed faster, like in the case of the Camera2 API, if someone manages to work around the issue so the dev team can get inspired by that code … 
Just my 2 cents to the issue.
I think the fingerprint reader only stores slices of your thumb, and then checks if the presented slice is somewhat like the ones in its memory. Because I unlock it quite reliably by just touching the scanner with my thumb, no movement. And I registered my thumb by taping different sides of my thumb on the scanner. No way it has a full print of my thumb, and yet it works (quite well for me).
That been said, I did meet the aforementioned “too many attempts” issue once, and that was clearly my hand touching the scanner while I tried to pick the phone up. Once I had it firmly in my hand it was locked.
This means you need to keep clear of the fingerprint scanner at all times, except when you unlock it or use the power button.
(For the record, FP4, no case, latest patches as of today, to unlock I have to click the power button and then scan my finger. Usually this means click on button, screen turns on, raise thumb, place thumb back on button, done. Works 90+% of the time.)
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To be clear, I don’t have the issue that the scanner doesn’t recognize my fingerprint(s), the success rate is somewhere in the ballpark of 95%+, the problem is that the scanner seems to think every part of human skin should be scanned repeatedly if it doesn’t get a successfull scan immediately.
I can press it against my arm and it will happiliy trigger “too many attempts” sometimes.
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Pretty much the same here. Most of the time I can get it to work, unless I’m careless in the way I place my finger on the button.
I’ve chosen the option to push the button first and then touch the sensor; that eliminates accidental readings when I don’t want it to even try.
However placing the finger is really important. In that respect, using the soft protective case is not a help, because the hole for the power button is quite deep and narrow, so I found I had to make a determined effort to quite firmly press a recognisable part of my fingertip against the button. In addition I have a leather flip case which makes things even more susceptible 
I have to hold the phone in exactly the same way as when registering the prints. I can never get it to work if I leave the phone lying on the table for example, instead of picking it up, the angle of the finger isn’t right.
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Did you reset the fingerprint after adding the cases ?
Yep. In fact I sometimes delete the oldest registered print (the oldest one is at the top btw) and create a new one, especially after a repeated failure, or new conditions (the skin reacts to cold, for example).
When viewing the list of registered prints, one can touch the reader and see which of the prints was recognized (it gets highlighted for a moment). That’s handy for knowing which can be deleted first.
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Seems that lineageos 20 solved this for me. It used to only recognize 1 finger, now it works on both fingers i use.
That’s after you enrolled your fingers again, right? Did you try to enroll your fingers before you switched to LOS20? For me this issue only happened about a year ago or so. A software fix solved it for me back then.
Yeah many times, worked for a day and than they stop working again. Feels like lineage is not polling the sensor 500x in 2 seconds so i dont get to many times error.
I can confirm that this problem exists on LineageOS20, I have a brand new FP4 with LOS that I flashed last night and after enrolling new fingerprints the problem is still there and presents in exactly the same manner as the stock OS
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Wow i have the opposite experience. Fingerprints take directly, on both hands where before it would only read one finger consistently. Guess i was lucky or you unlucky? Guess the fingerprint scanner still is a coin flip.
I have no problems either most of the time (stock FPOS). The soft case makes things harder because the sensor / button is at the bottom of quite a deep hole. See my remarks above.
its not about not recognizing the finger, but that the scanner make multiple attemps within seconds without lifting the finger or whatever part of your hand you put on the scanner…
not a big surprise as it was reported “for Calyx” already since some time here
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Yes, and my point is that people are not placing their finger-tips firmly enough on the sensor for it to be able to read properly. I’m also fairly confident that you would find that it’s more difficult when using the soft protective case. The sensor is not very wide, it’s recessed in the phone, and even more so with the case.
So I think there are also problems arising from different ways people use, hold and touch the device.
I even can’t reproduce this issue if I wanted to. Testing sideways, soft press, wet thumb or a bit dirty. At worst the phone says that the scanner is dirty. No instant 100 tries. I think it has to do with how people enroll their finger. Otherwise it may be an issue the scanner has problems with certain fingerprint types. Then maybe test with a friend if he/she has the same issues. Because then it could also be faulty software.
I think we discussed this before and I still dont agree to this. We see so many issues that dont affect everyone, so just because you cant reproduce does not mean it does not exist. And again: the scanner should not try to scan before my finger is placed on the scanner properly or try to scan any orther part of my hand, because thats the issue. Like if just energy coming closer starts the Scanner. So yes maybe you are better in directly and fast enough putting the Finger on the Scanner, still it does not mean others do it somehow wrong.