If their shells are eroded but do not have holes, you can use Sally Hansen Hard As Nails to paint over the shells and stop any immediate erosion. Take the snails out of the water and place on a shallow bowl or plate with water on it. Dry their shell carefully with a paper towel, and paint the shells. Be sure to avoid getting ANY of it on their bodies.
If their shells are just small and in poor shape, but not too eroded, add liquid calcium to your water and put in a few plaster vacation feeder blocks. Be sure you have a high pH.
The best calcium supplement is Kent's Liquid Calcium, used for marine tanks. Add the suggested dosage for a marine tank to your freshwater tank. I recommend mixing the dosage with a few cups of water in a bucket, and adding that to the tank. Adding it straight to the tank can cause an overdose and kill the snails.
Good luck. Snails are pretty tough and have an amazing ability to repair their shells. Keep us posted. Photos would be very useful.