Don't know if anyone else realized this, but I think you need to beat a higher rank opponent to be able tor ank up in Ghost battle. I lost to a Tekken god character when I was about to rank up, then I played like 5 lower ranked opponents without getting a promotion chance. Then I faced and lost to another tekken god, while getting the promotion chance.
Then I faced ANOTHER Tekken god directly afterwards, and I got the promotion chance again, this time winning. The only logical conclusion is that you can only get the promotion chance vs a higher level opponent.