My phone reguarly charges only very slowly. Sometimes 10% in a night, this night -1 % even. I have read a ‘solved’ (sic) topic on it here: [solved] Stuck in "very slow" charging mode. But reading it did not help much.
I also thought it was a HW problem (several times reconnecting seemed to help, cleaning the contacts perhaps) but what does help in my case is restarting the phone. So far.
This is the 4th issue I have with FP3.
I am sad to say I stopped recommending the phone among friends.
I know from personal experience that some charger/cable/phone combinations lead to the experience you describe. Maybe you can tell us what you already tried, for example:
try a different charger
try a different cable
try the charger/cable combination with another (Fair-)Phone and observe if you see the same issues
try a data connection to a PC
In my case there is a Samsung quick charger that can charge the tablet it came with fine as well as my FP4, but totally fails to charge my wife’s FP3, which will charge fine with the same (thin) cable, but a different - and pretty much any other - (quick) charger.
It seems the USB C ports also wear out. I have 3 bottom modules, 1 almost new and 2 used. Its quite a difference when putting the cable in, its more or less loose in the 2 old modules. 1 of the old modules only charges slow (sometimes when moving the cable in the port I manage to see fast charge), the other one still charges fast, but cable tent to fall out of both quite easily and there is no debris in the ports. The cable noticeably clinches into place in the new bottom module.
I can recommend them, I’ve been using magnetic adapters as a matter of principle for years now in a range of devices with micro USB or USB-C charging ports, not only smartphones.
While there are cables on offer with the magnetic counterpart fixed to them, I would rather go with the sole adapter parts where you can attach any cable you already have.
Worth a note, too: They get produced by different companies in different product generations and are usually not mechanically interchangeable between different manufacturers or even different product generations by the same manufacturer, as far as I’ve seen so far.
So, test a pair first, then equip everything you want with the same ones and keep some spares for future devices .