Repairs have worked, excellent resource for snails above.
Try some Crab Cuisine or Shrimp Cuisine by Hikari (not shrimp pellets as they are just food made out of shrimp for fish) they are created with the needs of creatures which need minerals to make their shells.
What are you feeding the snail? A bit of blanched spinach is very good for them. You can do a flake of oatmeal, the long cooking kind and see if the snail likes that. This is a new find I will be trying in m tank this week. Not the kind with salt and flavor in the packets either. It has beta glucan which is good for people as well as snails I understand.
Maybe get a bigger tank when at all possible, those snails like to cruise around. You might be able to make a lid for it with plastic needle point base
they come already round
Plastic Canvas, Circles
or cheaper and you can cut to fit
Darice Plastic Canvas, Clear
Snails like to get out of the water to lay eggs or whatever so see if you can leave an inch of space at the top.
Try a bird Cuttlebone from the pet bird section with no colors or flavors on it.
Chop or break it into pieces put it in the tank it floats for awhile. It should float with the shell up and the soft part in contact with the water. When it sinks make sure the shell is facing down and the soft part up. If you have enough room you can add it in whole, sticking up like a surf board in the sand and the water will wear it away because it is so soft and it will create an interesting pattern over time.