First and foremost I assume your horse is healthy, sound, and pain free and there are no issues with back or otherwise.
Horses, much like people are left or right handed and without getting complicated lets just say your horse sounds like she is quite normal in that she does not enjoy going one way.
You must always be sure to lunge or work both sides of a horses brain equally. So when you lunge you should lunge to the left and right equally. When under saddle you should also work all three gaits equally so both sides are exercises. Do some flexion exercises, ask her to bend and flex softly for you both ways.
Bucking when going from a trot to a canter may simply be that she is unbalanced and needs plenty more walk, trotting, before asking for too much canter work.