Nerites often get bunched together as if a horned nerite and an olive nerite are the same thing but just a different color and size. They aren't like dogs that have been bred to be different in appearance, and I personally believe that some species do better in one environment more than another. I've tried olives twice, both times they died within a few days though they were live and active after purchase.
I think there are a few likely problems:
1) you really don't have enough food for them (keep in mind they won't eat some forms of algae, namely some of the harder varieties, and BGA is toxic for them, at least in large amounts)
2) they aren't true freshwater snails, they are likely shocked from being taken from a brackish environment, where they are shipped to big box stores which dose their systems with salt (yes, freshwater fish at stores like petco/petsmart are kept with some salt in their water, so I've been told) and then suddenly they are moved to a salt-free environment.
3) Even with salt, and food, the stress of constant relocation can kill them.
Buy from a reputable online source, if possible get a few species, (track, zebra, olive, etc) and introduce them to your tank slowly over a few hours before dropping them in. Past that point I have no idea what's wrong.