I had problems getting my horse to canter too, when I started lessons last year :). I think it's quite common and it does get better as the clarity of your aids improves.
A couple of things that I found helpful:
Before you ask for canter, make sure the horse is doing an active trot, not a 'ploddy' one. If he's already motoring along in trot when you sit and ask for canter, he will understand the instruction more clearly.
As you stop rising and sit in the saddle, sit deep and don't pull on the reins. If they are already nice and short when you're trotting, you don't need to shorten them any more. If you shorten them, it might feel to the horse like you're putting on his brakes.
Use your voice as an extra aid - lots of praise when he does a good fast rising trot, and change to 'canter!', as you apply your leg aids. Keep praising, clicking your tongue, and keep your leg on if you feel he might go back to trot again. I have mobility problems so I find it hard to give clear leg aids - and voice is key to me being able to tell my horse what to do!
Are your knees relaxed? Are you holding your breath? I do this sometimes
- and the tension transfers straight to the horse!
Because my legs are not very strong, my instructor gives me a whip so that if the horse is not responding to my first leg aid, I can use the whip instead. She tells me this is fairer to the horse than applying a lot of seemingly meaningless and confusing kicks. She was very careful to show me how to use it correctly. As my riding improved I found I needed it less and less. These days the horse responds just to my voice, and I love it when I praise him for being good, and he whinnies back to me
!
I am still a novice so you might already know all this - but I am sure others will be along in a minute to give you (and me) better tips! :)