David,
the channel separation spec is a measurement of how much left and right signals are separated (the higher the better for performance); and not relevant to your lower gain issue directly. the output voltage of 5mV is the big number for this discussion; and that robust output should give you plenty of 'gain' and 'life' in the music....considering a typical MM 40db input phono circuit gain.
looking at your connections both on the receiver chassis and cartridge all appears to be fine. color coding on the cartridge pins looks right. nothing obvious to quibble with.
next we consider things which might not seem to be relevant, but could turn out to be.
is anything mechanically loose? are the cartridge screws snug? (don't over-tighten). is the arm firmly attached to the plinth? these should not be involved in your situation, but you never know.
now we consider if maybe that the cartridge is not performing properly? maybe the stylus is loose or a piece is missing? unlikely as you would hear lots of distortion, not just lower gain. but get a magnifying glass and have a look. maybe clean it if you know how to do that. make sure you run any brush the correct direction.
you could also play around with grounding your cartridge or receiver. sometimes that can be a big deal. it's not logical it could be significant enough to cause your gain problem, but it's just a thing to consider.
if there is any way to substitute another cartridge or receiver/phono stage, or even a different phono cable, or test the cables, then these are possible issues......but very unlikely.
honestly it's a head scratcher and your gear looks straightforward and set up properly....to me.