Here is what I learned the hard way, and might have had something to do with yours dying.
On the limpets and chitons, you definitely need to individually stick them to the rocks. I had a big order I got this week, and was trying to get everything in the tank. I didn't place the limpets and chitons on the rocks, and put them in the sand bed. The Nssarius snails devastated the limpets, and might have killed both chitons as well. I did get them later and put them on rocks, and got a few of the limpets on rocks and glass, but not before the N-snails had killed over half of them.
Stephen