She does know voice commands. And It's basically a large oval, but the ground is a little uneven.
It's also only in one direction that she can't canter
She can't canter up hills, on the flat, whatever.
I'm going to try her in the sand ring tomorrow, if it's nice out.
It is much more difficult for them to canter in any other shape than a circle when they are first getting the canter/relearning it.
Next time you ride you may try trotting a circle (smallish) and then as someone else said sit up straight and ask by putting your outside leg back. If she speeds up her trot slow her down and repeat until she gets it. When she gets it praise her lots and lots.
Maybe you should get someone out to check her back etc as that can cause problems with cantering. Just a thought.