I have the same issue - vanishing amano shrimp. I have a 30 gallon planted (not heavily) community tank. Fifteen neon tetras, 2 Corys, one pleco. I added 10 amanos about 3 weeks ago and for the first few days, I could find 5 or 6 but sightings declined and now I haven't seen any of them on the last week. I've looked everywhere, including the filter, even though I have a sponge filter on the intake.
I've read on other forums that they can climb out of the tank but I don't see any carcasses. Anyone have ideas on where they went?