Horses are just like people and will tend to favor one side or the other and will be more balanced and supple going in their favorite direction, but are off balance and uncomfortable going the other way.
If you use a round pen, spend more time going in the direction she doesn't like, and make her do roll-back turns from her good way to her bad way often. Don't just keep going the same way more than a few rounds before making her change directions. Maybe 2 laps her bad direction, 1 lap her good direction, 3 laps bad direction, 2 laps good, and change the number of rounds each time you turn her.
This will help her become more used to using the weaker muscles and balancing better to her weaker direction. As she becomes stronger and more supple going her bad direction, she should start picking up the correct lead as that will become the more comfortable way to travel.
The same can be used with lunging on a line, if she responds well on the line. You may have to be more firm in making her roll-back onto her leads on a line though.
Good luck and just spend lots of time working her weak side.