It's an old story that starts with a new horse owner who's chosen an inexperienced horse to train the way they like. I have long noticed, however, that those clever beasts align themselves very quickly with their rider's weaknesses. No one's immune to this phenomenon. My youngsters are smart & sensitive, so as much as I am training them, I am also self-assessing regularly to see if there's a link between the current issue and something I'm doing incorrectly. One of the first places to look for gaps is in one's own position.
Try this: lie on your back on the floor and raise one leg, then the other. One leg will be smooth and easy, the other will pull and stop much sooner. Quite likely, the connection between your legs and core are asymmetrical, and that will affect your work in the saddle. You might hide this on a trained horse that only requires subtle aids, but to teach a youngster, you must be prepared to adjust.
A rider works on their position every time they sit in the saddle.