So what you need to do is kill yourself 3 times. Then, once the first host leaves, every host picked after that will begin to leave until the lobby is empty. Then, you need to prestige, back out to main menu, go back to multiplayer, prestige again, and then watch the video again. If all these steps aren't followed, the level wont stick. Trust me, I did it to 15th and then it deranked me back to my original level because of not watching the video again after prestiging.