When ever I notice a step in my training stressing the horse or getting no result at all(mental shut down wish a refusal to canter would indicate for me) I go one step back and bring them along so they gain confidence and maturity toward the next step. It very well could be she's just not mentally or physically mature to canter in a small circle. And that's fine, 2 years olds don't need to be doing much of it anyway.
My filly just did her first canter in the round pen the other day without me asking, she was feeling good. So I asked her to trot again and then asked her for a canter. She showed me she was ready but then I had to make sure she understood it's only something she should do (when we are training) when I ask for it. We did one lap each direction and then went back to our walk/trot work.