Won't pick up Right lead

Sandy, Brennen's buckskin mare picks her left lead consistantly. But will not get her right lead, even on a small right circle. She's always kinda been like this, but because Brennen didn't lope her a whole lot, I never really realized it till we started working on getting ready to show. I even got on her and tried. I got it right 1 time out of about 7. Brennen got it right a few times, but we need to really work on this. Any suggestions?
I am taking him for lessons this week. And have talked to the instructor, but wanted any suggestions from you guys.

My mare did this on a continual basis... She was "left hooved" so to speak. So every time we loped/cantered she'd pick up her left lead even when I wanted her right lead.

I had someone watch us, and they pointed out what I had thought... because its a preferance, she drops her right shoulder and simply cannot pick up her right lead.

To fix this, I picked up her right shoulder, and also made her run slightly crooked (I cant think of the proper word right now ) so that her right hind hoof would be in a better position to begin leading when I asked for her to lope. Also, making sure you are asking for the lead while turning rather than running a straight away will make it easier for her.

Have him collect her some, wait till he is beginning to bend into a right turn (like at the end of the arena), and touch behind the girth to move her hip away (thats how I cue anyways) and ask for the canter, all while raising her right shoulder.

See if that helps and let us know how it goes

Yeah, I'm sure the horse is dropping his shoulder which makes it nearly impossible to pick up te right lead. Merikle's idea on getting her to raise her shoulder is right, and, what ever you do, don't let her canter on that wrong lead! Stop her immediately and make a big deal about it, backing her or bumping her face orsomething to make her realize that's not allowed. When she does get the right lead, make sure you drop the rein and let her go. Don't bump her face and praise her a ton!
Also, I would longe alot on that lead to help her build up her muscles and her topline, that will allow her to pick up that right lead and canter in a collected manner with her head level. Good luck. It just takes consistent work, but we also broke our horse of this bad habit.