It could be the rider. If he canters by himself and on a lunge line with NO rider, but then can't canter WITH a rider, I would be suspicious. Check your position. Make sure you are sitting back, not forward on the horse's forehand. A horse needs to lift and lighten his front end to canter, and 70% of his weight goes onto the hind quarters, so that's where the majority of your weight needs to be too!
Here is a quick article that may help you as well.
Horse Won't Go
A common question I hear is what to do about the horse who doesn't want to go; he's dull, lazy, stubborn... and other creative words people use!
I like to examine all horse behaviors by categories. If you think of an engine having three systems; air, fuel, fire, you know that if any one of these systems is not working, the entire engine will not operate properly.
Horses also have three systems that operate in a specific order; the RESPECT system, the IMPULSION system, and the FLEXION system.
Therefore, if there's a problem in the second system, IMPULSION (won't go), look for the problem in the first system, RESPECT. I'll save FLEXION for later.
Getting the "go" right
Most people are told to kick a horse to go, which is ridiculous when you think about it from a horse's point of view. Imagine if you were kicked in the ribs on the way to the dance floor... what would your attitude be towards that dance partner? Would you even want to go?
By using four distinct phases of polite assertiveness, the horse can quickly become a willing partner; happy to take our lead to the dance floor.
Phase 1 – Smile with all your cheeks! Take a long focus, stretch your hand out in front of you with the reins, and tighten your cheeks. If the horse has not moved forward from this suggestion, continue through the phases and be ready to release as soon as there's forward movement.
Phase 2 – Squeeze with your legs, starting at the top, then all the way down to your heels (turn your toes outward to make smooth contact). This is not a strong squeeze. If you are straining or getting cramps, it's too strong! Remember, a horse can feel a fly land on him.
Phase 3 – Smooch while holding the squeeze, do not let go with your legs.
Phase 4 – Spank. Start by spanking yourself lightly slap your shoulders from side to side with the end of a rope (like the 12' Lead section of the Horseman's Reins on the Natural Hackamore).
Allow the rope to grow longer and keep up the flapping rhythm until it starts touching your horse on the sides of his hindquarters, letting it get progressively stronger if he has not responded.
The moment your horse responds, release your legs, quit spanking, and keep smiling. If he stops or slows, repeat the phases again. Always begin with Phase 1.
• Probably the most common mistake is kicking out of habit, quickly losing whatever respect you just earned, so really keep a watch out for this.
• Another mistake is to keep squeezing and/or spanking after the horse has made the effort to go forward. This feels unfair and confuses the horse because they don't know what the right behavior is.
• Be sure to put slack in the reins. It's a common habit to put contact in the reins when the horse goes forward. This is sometimes enough to confuse a horse trying to do the right thing.
Begin each time at Phase 1 and be prepared to go to Phase 4.
Finally, be sure there's enough 'life' in your body when you ride. Think about how fast you want your horse to go and bring up enough life in your body to stimulate that... then let the squeezing, smooching and spanking support it. Your horse will learn very quickly how to get in tune with you.
I can guarantee that just reading about this will not fix the problem. Go out and play with your horse. I'll bet you'll both find a new level of respect and communication for each other. Happy dancing!