How good are your simple changes, and your changes of rein? It's possible to overbend the horse. Also getting really good at loping on each lead very straight.
I would work on walking and trotting at the two-track, head on straight, and then changing rein to two-track the other side in the center of the arena. Make sure he's moving forward. Then gradually decrease the two-track until you can just shift the ribcage at will.
Then I would work on your simple changes while changing the ribcage over the pole. You can start out loping left lead, trot two strides over the pole while changing the ribcage, then lope right. Then get it to where you take one step, and a half trot step to change the lead. Often time I will aid with a light tap of the crop over the pole to help intensify my leg during the change of ribcage.
Finally you want to do the flying change. Remember to keep it moving forward, don't get too sucked back and collected. Lope over the pole, change your bend, and add the crop. If he doesn't get it, simple change and try again. Using the crop behind your leg, if you've used it at the trot, will give him more incentive to lift the behind. He might even buck during the change, which is fine, you just want him to think change the back with the front, not front to back.
I would also try and two-track him at the lope, then completely change the bend to two-track the other side over the pole. This takes a little more finesse, as you can really get too much in the horse's face, and you also run the risk of being too bent. Eventually you want a straight change.
When you get a lead change, try and stay as straight as possible after it, and stop before you have to turn. Give him a big reward. You want to think of making the straightest line possible through the center and turn at the last minute, otherwise your horse will always think he has to turn to change his leads.