If its not the teeth, make sure there isn't any thing else wrong...ulcers? pinching? ill fitting saddle? soured? afraid of something?
I have a raring problem with one of our horses...he hates gates (thanks to a previous owner) If you know you'll be entering one, look out, its rodeo time! After a couple of years, he's doing much better, but it will always be an issue. Horses just don't seem to forget! Primarily...patience.
(I've even seen one who hates men in cowboy hats!)

Thanks for your help. I'll try that.