Horses are like humans, they can be left handed or right handed. They tend to have a favourite lead, and if we don't have them 100% balanced they will pick up that lead.
Also the rider factor, you may not realise that you are unbalancing him and making it easier to pick up the right lead than the left. Make sure that you are asking for the leg with your seat, and not just kicking randomly and hoping for the best.
When I have a horse that struggles to pick up one lead, I will ask for the transition from leg yield off the inside leg, on a 20m circle. This puts the horse's body in a position that will allow it to easily strike off onto the correct lead, and stay in balance. If the incorrect lead is picked up, I'll bring the horse back IMMEDIATELY, re-set, and ask again. Not until the correct lead is picked up, will I allow the horse to canter off.