Let's start with the basics of fingerprint evidence since it can be a bit tricky to spot sometimes. The main thing to understand is that not every ghost will leave prints when they mess with the lights, and even when they do there's a few important things to keep in mind. If you're hunting an obake for instance theres only a 75% chance they'll leave prints behind with each interaction while wraiths won't leave any prints at all no matter what they touch.

The prints only stick around for a minute or two depending on your difficulty setting so you'll want to check pretty quickly after any activity. Light switch prints are actually smaller than the ones you find on doors and windows, which makes them extra tricky to spot even with your uv light positioned just right. Something interesting to remember is that when a ghost touches the same spot multiple times it might not refresh the fingerprint visual and sometimes they can flick lights without actually touching them especially during ghost events. Just because you see the lights going crazy doesn't always mean there'll be prints to find.

While technical issues can pop up now and then most of the time when you're not seeing prints its probably down to one of these mechanics rather than a bug in the system. Just keep your uv handy and remember to check quickly after any suspicious activity!