Make sure you are being clear in the transitions by using the correct aids and sitting straight in your body. Avoid tipping to the inside or outside.
Focus on the quality of the canter. To gain more impulsion your horse must be going in a balanced, rhythmical way while being reactive to your aids.
If your horse falls to the inside of the circle, use more inside leg to support him and try some transitions on a straight line to correct him.
Check that the connection in the rein contact doesn’t change during the transition. He may need adjusting, through the use of a give and retake or even a scratch on the wither, to help him stay in balance.