I am winning many matches in a row but my rank does not rise and when I lose it falls a lot
The first post in Dr. Menke’s matchmaking feedback update details everything out. If you still have questions after reading, feel free to leave a comment. I would read over the last few pages as well though as it is highly likely that any follow up questions have been answered already 
https://www.halowaypoint.com/en-us/forums/58b8518e005f432381ab99fbcaf931e0/topics/matchmaking-feedback-update-%e2%80%93-may-21/7d3f6e81-5f9e-44f7-b5cf-30f85d13f320/posts