Originally Posted by TheLovedOne
I think that leaning forward causes some conflict in their minds because the weight is going in front of the drive line.
unless your sitting in front of the wither, you aren't in front of the drive line. But leaning forward will possibly impair their scapula movement, which often times makes horses unhappy about cantering.
Shim your saddle (even if you think it fits properly, shim it). This will provide more freedom of movement, more than likely leading to a happy canter.
This is assuming the problem isn't based on poor training.