Here is what killed all seven of the clams that I had collected over 5 years. Have you ruled them out completely? I rarely ever saw them on my clams, the species that I had must have preferred other snails. I would however see them all over the glass, sand and rocks during the day and night. Once I went clam-less and basically snail-less for several months they all disappeared and I haven't seen any in the last year.
Begin doing regular water changes, thats obvious. My other recommendations are to give them a freshwater dip, then remove them to a quarantine tank and do daily water changes on it with water from your main system. Replace the water that you remove with newly mixed saltwater. Then just wait and see if they improve.
The astrea snail in this picture is actually still alive and well in my tank a year after the clam genocide.
For nearly a week before each one died they would look like this.
