I presume you mean Astraea snails? What was your bad luck? If you had 25-30% survival after 6months your luck was about normal. These animals do not survive in reef tanks for reasons well-known, they can't right themselves when they fall, as they inevitably will in a place where you can't reach them. Or, like most, you will tire of righting them.
If it was true turbos, Mexican turbos, they have less survivability because they are often collected from waters much cooler than our reefs, so we get the wrong types. Even the right types, hasty acclimation often shocks them to where they die within 2 months (yes, it can take that long).
Spend the money and get Trochus snails. Also, don't overstock. People tend to get 1-2 per gallon and that is enormous overkill. Trochus is one per 5 gallons or so, astraeas no more than one for every two gallons.