Have you had someone look at your rowing technique?
Are you sitting on the seat correctly?
I know someone who was experiencing some difficulty that stemmed from how they were sitting on the seat.
You are correct a difference side to side is not good.
You are seeing the issue at the knee, but this may not be where the cause of the situation is.
It could be something upstream or down stream.
Have you encountered any problems with your hip, knee, or ankle with other skills?