How are you asking her to canter?
I find this to be the most helpful method:
Get to a slow, relaxed, sitting trot and ride along the rail. I find its best to ask for canter at a corner. So as you approach the corner, (for getting left lead) slide your right
leg back behind girth, and left
leg a bit forward and do voice command (click, kiss, etc). Give with your inside
rein, to allow the horse's left leg to lead the motion of the canter.
If your horse DOESN'T pick it up right away, chances are she will start doing a really fast trot, since you have already pushed her forward when asking for canter. As well chances are you will be unbalanced and unsteady. So instead of asking right away again, go back to a nice sit trot again, or, come fully back to walk and THEN go back up to sit trot to ask for canter. It's much better to ask for an upward transition when the gait you are in is controlled in itself.
If your horse picks up the WRONG LEAD, apply half-halt, sit back and ask for the trot again. Trot around for a few strides and collect yourself, getting back a nice, even pace. As you approach the corner, ask again. Make sure not to hold back your horse with your reins. Open up your seat and give with your inside rein, allowing for forward movement. You might even want to move into a half-seat position.
Once you get the canter, ride it out, relaxed and balanced and then come back to walk. Sometimes its nice to end with a cantering circle before coming back to walk. Always finish off well, with a smooth transition down to trot or walk ... always praise when she gets your cues.
Hope this helps. Good luck!
*Oh, and also -- she may be acting lazy and dropping her left shoulder, making the canter sloppy and uneven, resulting in the wrong lead. When you feel her heavy on the forehand and dropping her left shoulder, "pick up" her shoulder with your inside rein, bringing it up towards you, and bump her with your inside leg.