It may just be a matter of how much pressure you have in your outside rein, coupled with the pressure of your inside leg.
Play with different pressures. Make sure you keep your calf on to push him forward at the same time, an all too common mistake is to focus so much on your inside thigh/knee and outside rein that you drop all other aides.
Do some single loops (i assume you know the exercise since you are riding dressage?) These help Bailey greatly, as he often ignores my outside rein. :) Also do some serpentines and bend changes. Half halt in the middle, and ask for the bend change (new inside leg pushing to outside rein).
Make sure that you start each ride sending him forward, getting him going with his hind end. I seem to be adding it to every post, but I *really* do believe that 90% of problems arrise because the horse is not pushing enough from behind or is not pushing evenly.
"The white horse moved like a dancer, which is not surprising: a horse is a beautiful animal, but it is perhaps most remarkable because it moves as if it always hears music."