Snail Farming is interesting and lucrative but like most other types of Livestock farming, there could be some other Challenges and some of these could be some of the reasons why your Snails may be dying.
1. Flies: This could be as a result of Leftover food and rotten fruits inside the Housing, Once the Fruits decay, they begin to Smell and this attracts ants inside the Housing, the Snails become uncomfortable and could be dying gradually.
2. Sun/Heat: There Should be a great Consideration for ventilation while choosing a housing system for the Snails, Consider building a Structure with nets and then a good shelter which will help the Snails against direct Sunlight or sun infiltration, don't also deprive them of air while trying to do this so that heat won't roast them.
3. Water Logging: You need to be careful of this, Snails are not fish, they need water but don't live in water, Once the Housing becomes waterlogged, the Snails might get overdrunk and loose potency overtime.
4. Predating Ants: Ants as Predators are also another enemies of Snails, ensure you do a good selection of Soil, Site, and treat accordingly.