If the horse is very lateral in their gait then it’s more of a challenge for them to canter. But if you see an easily taken canter in the field then you know the horse can perform the gait. You just have to figure out how to cue it in your riding style.
So, let’s ask about style. English, Western, or something else? Do you have a favorite trainer or training program that you use? If so research what that trainer/program suggests and give it a try. Many Walkers coming from “Walker Country” are strongly discouraged from cantering until their gait is “set” (whatever that means). So you will might have to first “un-train” the horse from fearing punishment if they canter and then work on the best set of cues and aids to perform best for you.
Do you take regular lessons? If so, ask your teacher. If not, maybe you should consider taking a few to, first, ensure that YOU are not the problem with your technique*. If that’s the problem then the solution will be quick. If you’re not the problem then the teacher ought to be able to help you train the horse to assume the canter. If this fails, then go find a good trainer and engage them for a month to teach the canter. They need not be a “gaited horse trainer” but if you’ve got a good one around you then use them.
If at all possible I’d get someone to video you and the horse as you next work with them. The video camera does not lie. It has no regard for your status, feelings, or any other external element. It truthfully records what it sees. This can be very bad for a rider’s ego but is very good for the horse’s training. I most strongly recommend it.
Good luck in your program.
G.
*Something I see a LOT is the rider with a weak seat who cues the canter, a young horse will “enthusiastically” strike the first couple of strides, the rider becomes unbalanced, then leans back and checks the forward movement, and the horse drops back to the gait or walk. Then rider will “rinse and repeat.” This happens a few times, the horse gets tired of having their mouth abused, and they refuse to follow the cue. This is not a universal pattern but is an EXTREMELY common pattern. Hence my advice to find a teacher.