Strong Horse on Just Left Rein?

well, it could be health related... Or it could be something else? I'm not sure but maybe you could retrain him on the left rein? Like do ground driving or something. And work on the left a lot. Or maybe put his halter on underneath his bridle and turn him to the left while pulling on his left rein to remind him? I'm not sure cuz it is weird that it's just that side... Anyway good luck!