[iMore.com]

Touch ID can sometimes be a mixed bag and if you find some prints just not working, it may be best to delete them and start over — assuming you've already tried retraining Touch ID with no success. To wipe the slate clean and start over, you just need to remove all your prints. Here's how:

Full story from the iMore Blog...

[Jaguarr40]

I don't think prints go bad and rarely in the commercial Biometric Finger print readers go bad either. I think like I do even over training the print with some success, I use a micro fiber cloth and gently without pushing down on the home button just wipe off any grease, grime, dirt or anything that might be built up. I wipe mine off at least 2-3 times during the course of a full day and when I put my every day driver to bed at night when I hit the sack, I wipe the screen and Touch ID pad.