I had a problem like this that I was able to electronically compensate for on different channels and different servo slow speeds on my 12x and BARF.

The culprit? If you see the image below (it just happens to be an 8411) the servo arms from JR can be very close looking from one to the other if you aren't careful at matching them. One servo arm will be different than the adjacent arm, eg.

My dumb arse was off by one hole and finally (after hours, at the field) pulled out the servos and saw my mistake.

I have no excuse, I built and put mine together. Flaps can be a pain in the butt...[:@]

Raf