Not sure about the Fluval, but with Rams in there I would make sure that you start with reasonable sized shrimp. I bought a mixed-size group of CRS and the large ones I still see regularly. The Rams (as well as Tetras etc) have quickly learned to ignore them.
However, the small CRS I haven't seen since the first day. Doesn't necessarily mean that they've been taken - they could, very sensibly, be hiding out until they grow bigger - but there's also a possibility that they've been eaten.
Finley