Basically as you win games, the system thinks you are getting better. This results in better opponents. The problem is, when you need to win 2-3 games in a row to get to promos, then win at least 2 more. Because you won several games in a row, the system compensates by putting you against players that are much better. EG starting at 100, you win a game you are at 110. You win a 2nd game you are at 120. Your third game 130, and you get to promos.
The system now thinks you are much better than it originally thought so it pits you against people 150 or higher because it thinks you are at that level. Result is lost promos almost every time.