Lots of transitions help the horse respond to the signal more responsively
I use my inside leg instead of my outside leg and they always pick up the right one
Sit, outside leg back, inside squeeze, give him a tap if hes not responding to your leg!
When picking up the canter from the trot, sit 2 beats, make sure the horse is bent around you inside leg which should be at the girth, then add your outside leg behind the girth
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.
Inside leg on the girth, outside leg back and lift up to canter