It will match you up against higher leagues before promoting you for a good number of games. I leveled up silver->gold->plat in retail and that is how it announced the impending league promotion to me every time. If this didn't happen yet, you might have to improve your win ratio. I kept crawling up the leagues with a stable 60% ratio.
btw. If there are some EU-b.net players lurking about, feel free to get in touch for some games (Gnaddel.251