Sorry, but saying snails are detrivores is incorrect. Some snails may be detrivores, but most are either carnivores or herbivores. "Turbo" snails, and probably the strombus (depending upon the species), will not survive on "detritus." They may be consuming algal films that are not readily visible (especially the strombus), but a lack of visible algae will often result in a die-off of number of snails.
What can you do? Well, you can increase your fish feedings, providing more nutrients for algal growth. You can increase your lighting - again increasing algal growth. You can buy some new rock, and rotate out some old rock (rotating the old rock into another well-lit aquarium free of snails to allow it to grow some algae is also an option).
Finally, you can also try feeding the turbos dried sheet algae. I have a large turbo that regularly comes onto the glass at night to eat the Seaweed Selects that I put in an algae clip for my tang.
FWIW,
Kevin