First, have your trainer/vet check her. She could be having issues with soundness/comfort which inhibits her to pick it up. Such as your saddle could fit her wrong (even if it fit before, she could be getting more muscle, or her back could be dropping, etc that can change the way a saddle fits). She also could have tweaked a muscle in turnout or in her paddock which could be sore now.
If all of those are fine, then work her on lunge or in turnout to increase the muscles so it easier to pick up the canter. Also, when you are riding her, start out with limiting yourself to two good transitions to the right lead for a week or two, and end on a good note. Then, every week or so, increase the number so she also gains muscle by doing this, and praise her whenever she picks up hte correct lead.
Also a side note; this is a common problem that I catch myself doing that I don't know if you are doing it, but many of us tend to lean forward in the transition to encourage them to go forward and/ or to check the lead. This is a simple problem to fix once you are aware of it