Aquariums: Mystery Snail dead a week after getting it?

Last Friday I bought a pair of new mysterys for my tank, I already have 4 mysterys and about a dozen shrimp in the tank. Tank has been running for a couple months now and is fully cycled. Tank gets fed algae wafers daily, plus there’s a block of snail brick food that gets replenished in the middle.

Anyway, I bought the snails last Friday, and on Wednesday noticed one of them not moving off on the side of my tank. Sometimes they do that, but as a precaution I nudged him over near some snail food. Since then he hasn’t moved at all, so I’m assuming dead (but he doesn’t smell yet, hasn’t fallen out of the shell). So I’m leaving him there for now, but assuming the worst. What could have caused this one to die, with nothing else being wrong in the tank? Should I be worried about the rest of my critters? If it had died right away, I would assume shock or something, but this is a young-ish snail (about 1/2″ in diameter), and he was fine and rampaging around the tank for 4 days just fine.

Also here’s a picture of one of my ghost shrimp because I think it looks super cool.