It's very normal for mystery snails to clam up into the their shells if they get startled, bumped, annoyed, if it's a Tuesday, etc. They're just fussy. They can take anywhere from a couple hours to a couple days to come back out of their shell, so to speak.
I suggest buying a piece of cuttle bone (available in the bird section of any pet store) and breaking off a little piece that will fit in your filter. A small piece will lasts for months. Replace it as it deteriorates. This will add necessary calcium to the water and other nutrients to help your snail's shell stay strong and solid. They will not survive well in acidic or soft water.
The best treat to give your snailis a slice or chunk of zucchini a couple times a week. They flip out over that stuff, and it has loads of calcium, so it nutritious for them. They'll eat the rind and the innards. I do suggest scooping away the seeds, as they can be messy in the tank. Once you slice a chunk off, weigh it down with a spoon (I bought one cheaply and new from the store, so that it was sure not to have soap residue on it...), and let your snail have at it. Take it out after 12 hours, or it will start to make the water gross.
If you pop it in at night, by morning it should be plowed through.