More details? Size of tank, how long has it been set up? Any other fish in the tank? Whats the "normal" way of cleaning for you? Did you check the ammonia, nitrate, nitrite?
Most people will agree that neons are not hardy fish. I've never had much luck with them. Perhaps you could go for another type of tetra instead? Cardinals are same colours but more expensive, glowlight tetras have an orange stripe and they are cheaper than cardinals but are far more hardy than neons I've found.