Questions:
You say your console is inverted, so 1-24 on the right, 25-32 on the left, maybe it has to do with the way it was inverted. So the right side 1-24 is not working picking up groups on small fader is this correct?
I would first try to isolate if this is a problem of the channel strips or the grouping buses. Insert directly a test tone in the bus groups inputs on the patch bay and try to pick it up this way.
Also try to pick it up with the big fader by pressing SUB GP on the channel input.
Let us know how it goes.
Cheers,
Artur