I am having a similar problem with Ada, and my trainer suggested leg-yielding in and out of a circle, at the trot, like a spiral. Does this make sense? Trot (or jog I guess in your case!!) on an inward spiral from a 20m circle to a 10 m circle, using leg pressure to guide Duchess in, then push her back out with your inside leg. If she feels nice and "arc"-ed to the inside, then ask, on the way from 10m to 20m, for the left lead canter.
Seems like for right now, the more times you can get her on her left lead, even if it's only for a few strides, the better! Good luck, I am still working on it with Ada too!