Hi Anita, thanks for your question.
While any of those could be a contributing factor, this problem is one that nearly every horse I see has on one side just as we are right or left handed.

They may fool us that they’re flexing by over-bending the neck, but are they flexed the same direction in that first joint.
Isolating it with a straight neck is the best way to check.

I look forward to you trying the exercise with each of the horses and seeing if they can flex equally both ways with a straight neck.
Once they can, then you work on it mounted. And finally you also work on it in motion. This is something you can also ask for while driving.
It should preceed every turn. Starting on the ground, and then on his back will help it to come easier in harness where the rein-effect is slightly different.

Getting the flexion working equally both ways is the first step to improving balance and controlling the drift of the shoulders.