its probably a lack of food. They can live for several years, I have had some for 3 years now in my display tanks.
The collectors usually pick larger snails which are probably adults already (especially with the zebras and the red spots). I try and get them as small as I can to ensure longer time in the tank but they are not offered at specific sizes.
I did get in alot of juvenile bumble bee horned nerites and green spikey nerites which are pea-sized and probably quite young. The issue with this is that larger fish may be more apt to pick on them at this size.
Its the difficult part of dealing with wild caught invertebrates that don't reproduce in freshwater, lol. You get the good (amazing algae eating capabilities) and the bad (unknown age of collected species).
One thing I try to do is put large rocks into tanks that I don't house the nerites in to culture algae. You can also put them in a bucket in a windowsill in some water to grow the algae. I then just swap these out ever few days.
A single nerite can clean one side of a 10g tank in a matter of days. I usually recommend a low stocking density at first (1 per 10g or so) and see how the algae is effected as they willl starve rather than eat prepared foods sometimes. I have had moderate luck supplementing them with kens veggie sticks and with blanched zucchini, but they really don't do as well as they do with algae.
