I also had problems keeping Margarita snails this summer. A while back, I read somewhere that these snails are cooler temp. spp. so that's why I had so many die off when temps went up in the heat of summer. I had 15 or 20 from Paul's & now I have only couple left.
Margarita snails are cheap, but not if they just end up as hermit condos.
Have had much better luck with trochus & turbos & stomatellas (these last ones came in as hitchhikers).
JME
Anthony