Let me be sure I understand the problem--if you run a signal any channel, assign it to your main outputs (L-R), centre the pan pot, and make sure both L-R main faders are up, no effects, you only get one side? And if you send effects from that channel, you hear both sides? (I'm assuming you're using your FX returns not channels to return your FX.)
What happens when you solo--pre and post (your mixer does both)? Do you get the full signal then?
Does this problem occur in mono as well as stereo channels? If only stereo channels, are you using the left or right input (assuming a mono input)?--(should be the leftone).
What happens if you assign your channel to the ALT 3-4 bus--do you get both sides then? If so, what happens if you assign ALT 3-4 to the main L-R bus? Does it go back to one side?
Let us know what you find after all of this...if you haven't tried it already.
Or the old whack with the stick...your call.