Ask and see when they got their shipment in. Snails sell fast and they could have just gotten there and the disease they had dint kill them until you got them into your tank. I have bought four and they mated and had over 100+ snails that I sell and have been breeding the same snails ever since.
But if it has happened more then once you may want to see if you have any copper in your tank. Only tow things that could really be the problem.