When I had changed the tail lights, I noticed that at first, the top left bulb was not picking up... It would pick up before I screwed everything in, but afterwards, it would not pick up at all.

Try unscrewing the taillight, then leaving it off and in park and having a friend or someone step on the brake. I believe the issue I was having was just a pinched wire.

Also, overheating could be another reason for that...