First,try to start the galop in corners or circles. Be aware of your outside hand(left hand) because if it is more advenced than the right, you are not controling your horse back, that it has to be slightly inside than the croup. Also, be aware of your outside leg (left) that has to be more behind than the right. If this doesn't work, you can try this exercise that is use for racing horses that only know to start the galop at one hand. Do it in a corner, your horse has to incurving to the left (a bit exaggerated) and your burden to the inside, then try to start the galop.
Remember your initiating hind leg (cantering to the right) is the rear LEFT. If your horse does not take the right lead correctly from the start, his left hind is not in position to start properly. THis means ask him when it is the most probable that his left hind will come down first. For example, if you are asking from a trot and you ask when the right diagonal is DOWN, the left hind is also down. This is the one stride to late to ask. Get it?
The croup have to be inside, for example you can ride a travers :)
ask your horse to canter/gallop while making a right turn.