This happened with a patch of mine tonight. I actually have a couple of others with the same issue, I just decided to try to solve this one tonight.
Anyway, after I did the obvious settings checks, mono-sum, etc., still couldn't figure out why it would not spread out. I had 2 empty FX blocks at the end of my chain so I dragged one behind the amp in Ch A and Bang! There they were, both channels panned hard in both the left and right sides. I don't get it. But it works now.