Does your horse have trouble picking up the right lead without someone on him? You can check this when you are lunging him on or off the line.
If he has no problems and if the trainer had no problems, it is most likely your balance being off. Even if you don't have problems with other horses, it could be because they are stronger and more schooled. If your hip is tweaked or rotated in any direction, or if you are sitting more on one side than the other, it could be enough to cue your horse to pick up the left lead instead of the right, even when you are tracking right.