I'd suspect it could be food issues. I never had luck with either of them living ling for me either. Got any hermits? I'm thinking that nassarius move too fast for hermits and turbos likely have a pretty strong "trap door" and cn fend them off. If you have no hermits, I'm sure that its just those two snails are less hardy than the two you're having luck with.
Your Ph is kinda low too. Again, the bumblebee and cerith may have lower tolerences than the other two towards stuff like that.
"Life may not be the party we hoped for but while we are here we might as well dance!"