I would run a program called
CPUz. It has a memory tab and can tell you a bit more information about what you have installed (or if one DIMM isn't showing up).
To properly seat the RAM, firmly and evenly push the DIMM into the socket until the two locking tabs on each end firmly clip into place. Sometimes if you've had empty slots, dust can prove troublesome. It can be easily thwarted with a vacuum, though
