To be honest the rankingsystem is pretty retarded, i win 8 games in a row not a rank up, i lose 2, win one and i rank up to eagle.
Don't try to get the system just play enough and you will rank up.
It also has something to do with the points you score and the kills you make, the rank the player has you kill and if you get killed it also makes a difference wich rank the player has vs your rank.