Left lead? Are you sure? Usually OTTBs resist picking up their right lead, and prefer the left, since they travel to the left on the track.
Check your own balance, make sure your sitting even and square, and that you're not dropping your shoulder as you ask for the canter.
The best solution, IMO, is to teach her to move off of your inside leg in both directions. Once she does that reliably, ask her to move off your inside leg toward the rail 1 - 2 strides before you ask for the canter. This sets her up perfectly for the inside lead.