Don't just rely on algae to feed them. Unless your sure it's getting some of the fish food, I would add food that's more targeted to the snail.
I didn't used to realise they need their own food & lost 4 small ones pretty quickly. I now have a large one, probably 3-4 cm and I drop in a slice of cucumber or croquette every other day, 10 mins after I've switched the lights out so that hopefully the fish won't eat them all. Ive had this snail for over 2 years now. Which is think is quite long for a snail.
Blanch the slices first of coarse.