When a horse disconnects in the canter they have generally been thrown off balance. When you ask for collection, what exactly do you do? When she has tucked her nose in, is she evading the bit AND on the forehand or simply evading? If she is on the forehand, half halting will/should bring her back to you and begin to collect her. I suspect she is a bit uneasy to balance and may still be a bit under developed in her haunches. I suggest numerous transitions and circles. Uphill work will also do wonders as well.