In team slayer, it is hard to rank up unless you are the MVP and have a positive KD ratio. The positive KD ratio is a must and so is the fact that your team wins. If you win against a team of mostly lower rank players, you usually won't get much credit for that.
Your ability to rank up is based on the ratio of games you won to the the ones you lost. If you won twice the number of games you lost, theoretically, you can rank up 2 ranks at a time. If you have equal won and lost games, it usually takes 2 wins to rank up one rank. Now your situation sounds like you may have a 1:1 ratio or less. Do you ever quit games? Because quitting counts as 2 loses and some people think it is funny to go social and quit out enough times to bring their exp to zero. HUGE mistake because there is no way to recover from that. You'll never rank up if you do anything like that.
Hopefully this helps.
