The new ranking system is much different than the old, now you cant just simply win a bunch of games and rank up from it, from my experience lately I need to actually be high in the leader board to affect my rank at all, I'd assume it all depends on how many k/d/a and mvp you get over the other team and also the rank of the kills you got. Recently I've won about 8 rounds in a row without ranking and then lost about 4, I then won 5 games and the last 3 were carries so I ranked up. They did this to prevent or slow boosters.