The bottom line with nutrients is how much are you putting in v.s. how much you are taking out. If you are feeding daily, and running a HOB
filter that you don't clean for months, I would say it's pretty easy to have high nitrates. Just put all the food you feed in a week in your hand. Do you take this much nutrients out of the tank in the same amount of time? If not, then there is your problem.
As stated before me, the snails and crabs don't really do any "cleaning". The reason you have to replace them is either that they starved to death, or they were killed by hermits. I'm betting they starved to death. Nassarius snails eat meaty foods, they do not eat algae or detritus, contrary to popular belief. They are carnivores.