I've found the most common saddle-related reason for problems cantering is the saddle tipping onto the shoulders. I'd make sure the saddle is sitting in the correct postion first of all - points three finger-widths (about 2") behind the back of the shoulder. Put it on without pad and girth then press vertically downwards on the pommel. If it drops (ie the cantle lifts) it needs extra flocking (or padding) in front. I'm assuming the correct width gullet has been fitted.
Actually, the most common issue is refusing to canter on one particular lead. This is usually one-sideness in the horse, and the saddle tilting to one side or pressing down more on one side because of this. So the horse doesn't want to stretch that foreleg out because he knows his shoulder's going to get pinched. Again, corrective flocking can often cure it.
Hope you manage to sort it out :) Cavalrytales Blog