Take your outside leg back towards his butt and then put you inside leg on and kiss to him. Make sure he has some reign but don't let him go all over the place. DONT RELEASE PRESSURE UNTIL HE GOES! As soon as he starts cantering take your legs off and reward
Have a nice contact on the reins pull back a bit on both reins, but pull your inside rein(rein of the lead you are trying to pick up) back a little farther. Outside leg back a touch and squeeze foreward. Do all of this at once. Also, you may be giving him the right signal, but with anything you need to be direct with your horse. You need to let the horse know what you ask of him because he's paying attention to you for what to do next. Sometomes, if a horse doesn't go to a trot or canter on over 2 strides, smack him with a crop.