Have you been out about 10:00PM with a flashlight? That's when I go out looking for snails and slugs. That should tell you if they are the problem.

I have my flashlight sitting here ready to go.

I was never able to catch the snails/slugs eating my hostas. Then, one night, I pulled up a concrete paver near my hostas and found 3 nasty slugs hiding underneath. So, my point is, even if you don't happen to see them when you go out hunting for them at night, be aware that they could have hiding places nearby. I removed the pavers from the area around my hostas, as well as other debris under which they could take refuge, and haven't had any more problems with them.

Sluggo or Escargo - non-poisonous to pets or people and the slugs vanish. Don't have to find them, don't have to kill them, just sprinkle Sluggo around every two or three weeks and the slugs go to never-never land. I was out yesterday sprinkling it around the Solomon's Seal and now have to get some around the kirengeshoma before they turn it into lace.
It's the only way to keep hosta's from being damaged. I also use it in the vegetable garden - perfectly safe.