Why do my sister's Black/Golden Mystery Snails keep dying?

The tank setup is fresh water, 1 gal. with an air stone pump, aquarium gravel, some empty shells, and a fresh water plant. I clean the tank every week or two, depending on water quality.

There are two goldfish and they are doing very well. The problem is that her Mystery Snails seem to die quickly. The first batch lasted maybe a month and we’re almost through the second batch and it just seems fast (I think we’ve had this batch for about 2 months). I had goldfish and snails in a regular bowl setup (no pump) when I was her age and I don’t remember my snails dying this quickly.

I did a bit of research into it and I read that the Mystery Snails often die due to lack of calcium in the tank (to keep up their shells), but we have shells in there that they seem to eat for this purpose, so I don’t think that’s it.

Is it normal for these Mystery Snails to croak so quickly? If not, what could be causing them to die?