Have you noticed the angels picking at them? The issue is that snails from a petstore or rarely fed well so they come into your tank weakened. They need a consistent calcium rich diet of veggies or pelleted food (ken's catfish pellets have 16% calcium). If the angel is harassing them by nipping at antennae, its likely they are not feeding. Bridgesii are not algae eaters, they need food. Your pH should ideally be well over 7 to prevent shell erosion.