Maybe the angle is bad but it does seem like you don’t have enough space on both camera and fingerprint reader.
In my case (no pun intended) the glue on Anna Treurniet’s case came off. This meant I had to reapply it. First I tried with some glue, but that didn’t stick for long. I used some white spirit to get that off and used dual sided sticky tape. This worked great, but in the summer I want to wear my smartphone on my belt (with a Mitch 15). So what I did is I applied some velcro. Now I can detach the Anna Treurniet’s case, and use Mitch 15 on my belt instead. I really like the idea of having my smartphone cover protected while phone is not in use. Young children and all that can wreak havoc. Btw, in all the circumstances I never used glue or tape or velcro around the letters of the Fairphone logo. My logo is still going strong! I also applied the velcro in the middle, with the help of a scissor I was able to get the bottom corners velcro’s too (with very minor velcro usage). The top corner I use some sticky tape, to reduce the amount of space between fingerprint reader/camera and case. It works great. However, not exactly as I wanted, cause without the sticky tape on top, the fingerprint is too far away.
As for your problem, you might be able to get better result with re-registering your fingerprint, or using sticky tape to reduce the amount of space with your fingerprint reader.