My new reining mare, Selena, does this exactly the same.
Ask, Tell, DEMAND. Key to training lazy horses, assuming it's not a bigger problem.
Ask lightly, just a little bit of heels. No response, ask a little ahrder. DO NOT WARN HER MORE THAN ONCE. If she doesn't respond, make her think she is chicken little and the sky is falling. I mean kick, swat your leg, swat her butt, etc. Obviously don't abuse her but just keep adding pressure until she moves forward. When she does, instantly relax and put the "cruise" on. Eventually she elarns it's just better to go than get yarded on.
Pssh.I didn't pick up the wrong lead
It's called a counter canter...
...A very advanced maneuver.