what can cause touch id to fail?

Dirty home button? Broken home button? Cracked screen?

It's difficult to diagnose this kind of issue remotely.

Please read these support guides:

Use Touch ID on iPhone and iPad - Apple Support

or

If Touch ID isn't working on your iPhone or iPad - Apple Support

If you get no help, have your device inspected at your most convenient Apple Store or Authorized Service Provider: Apple - Find Locations - Location Selector

Regards

Giulio

Well, to my knowledge iPhones and other smartphones are not "in love" with sand.

But, as I told you in my previous reply, it's really difficult to give you an appropriate answer about this problem, remotely. It would be only a useless speculation...

Make an appointment at an Apple Store for a check. They will give you all the available options.

Regards

Giulio