I have had the exact same issue your having and tried everything, and the eventual fix was to restart my tank and go through a whole new cycle. I think what happens is the GHA and Cyano become such a dominant part of the biosphere that the other things we hardly notice start to virtual disappear. All the while we think the snails have plenty to eat because we see film algae on the glass, but still the trochus and cerith wither away while the crabs rock on. At least that is my take. it could be that the algae outbreak is causing the deaths or at least causes acclimation to be very difficult for these select few. Either way the issue is the algae imo.
Also I don't want to be of alarm to say you need to do a total restart. What works for me is to do so and spray the rock with hydrogen peroxide before placing it back into the display and then treat the process like a typical tank cycle, just far more cautiously and typically over a much longer period. You may find another method more suitable to your tank/style, there are certainly no lack of algae threads on reef forums.
I would also like to add a tidbit about acclimation. I skip drip acclimation and hold each snail to the glass at the waterline until they take hold. This is based on the idea that they slowly exchange water with their environment over the course of days and have an easier time doing so at the waterline. This way I get an idea of how alert/active each snail is and how long they choose to stay at the top (supposedly acclimating.)