I had that issue on an old MTB and it turned out to be the front DR was not aligned with the frame and it pulled the chain to one side or the other causing the RD not to index correctly no matter what I did. After realigning the front DR the rear came into alignment and I was able to index it again. I was sure that it had to do with the hanger before discovering the true culprit. And yes, it happened shortly after a good knock on the ground. Good luck