snails can be pretty illusive sometimes - they are fascinating creatures to observe.
have you ever fed them zucchini? my apple snails love it. I attach a slice to a veggie clip and place it low in the tank. Some times I'll see 2 of them hanging on it - one on either side, just munching away.
If you try it, it may take them a while to find it. make sure you replace the slice at least every other day and sink a new one in the same place so they know where to look for it. Between my bristle nose pleco and the 3 snails, they go through a round every day :o)