What were the rank of your team mates?
League's ranked system works by a MMR score. Frankly, it is designed to estimate your skill level and keep you at the rank it deems you should be in.
A Bronze 3 player with a MMR of Silver 1 will gain ~33 LP per win and perhaps 10 per loss. While a Diamond V player with Plat or Gold MMR will lose ~25 LP and gain ~10 LP.
It sounds unfair, but keep in mind that the game matches teams for a close average MMR. One gains less LP the lower his/her MMR is to his/her actual rank is because the game assumes that your team mates with the higher MMR scores that brought up the average **carried **you.
Generally, people who played in the previous season climb extremely quickly after promos because the former challenge and master tier players are emptied into Diamond and Platinum, so everyone is a few tiers than where their MMR scores are at. However, as you play and win more games, your ranked will eventually exceed that MMR score.
In order to raise your MMR: Don't lose. If you win enough games at a higher ELO, your MMR score will eventually go up.