[AlixMore]

I found using my index finger MUCH more reliable (my thumb works OK, but just)
I'm someone who can't type very well with thumbs either - so could be anatomy related, heck who knows - give your index finger a try - mine even works upside down. Have heard that callouses and/or scars can also be troublesome.

[InTheRed]

If my finger is wet it usually doesn't work. Otherwise it is fine.

[Just_Me_D]

Nope. Fingerprint recognition worked well for me on the 5S and is still working well for me on my iPhone 6 Plus.

[Evilguppy]

If my skin is too dry it's a problem, otherwise, no.

[thomasliveshere]

Mine has been pretty much near perfect. 9/10 times it recognizes my thumbprint. The other 1 time I'm usually holding my phone awkwardly or something.
Try registering your thumbprint again?

[BreakingKayfabe]

I only have difficulty when my hands and fingers are dry from the cold.