Having MMR Problems

Hi guys,

So, I've had about a 70% win rate in my past ranked matches, with grades of mostly A to S flat, and I just advanced from Bronze 4 to 3. (Thanks again, team from my last series match! You were amazing, and this Soraka always will love you.) But I just checked my MMR again, and it's still about 200 under the proper for the league I'm in! I'm wondering if other people are having this problem, and if it's indicative of something wrong with my gameplay? Any help's appreciated, thanks!

"2.95 a minute." -Gustbreath