"seem" to be good, is not really too helpful when we are trying to figure out what is going wrong. Please post numbers whenever possible. If your snails are dieing that fast, I would suspect it is the acclimation procedure being used or possibly a high nitrate count. Another question is to ask if you have ever used copper based medication in this tank. Snails are very sensitive to copper, but then again, I would think the shrimp would be dead as well.
You are adding liquid calcium, then you need to be testing the alk
and ph regularly. How long has this tank been set up?