ok, so im not an expert with african dwarf frogs as I have clawed. but the first thing you need to check in the water quality and check ammonia levels, nitrate levels etc and make sure that is normal. Any you do have alive I would quarentine immediately into a clean tank. In regards to the missing one - i lost a frog a few years ago and we have no idea what happened to it, no body and we looked outside the tank around the area and couldnt find it ... so we assumed it might have been eaten by one of my larger females.

how long have you had these for?