lp gains and losses are determined by winning and losing based on the enemy teams average MMR.
lets say your team is low, like 1200, and the enemy teams is higher, say 1400. if you were to win this game you would gain a lot more LP. if you lose this game, it was expected to be in their favor, so you would lose a little less.
it really matters who you are playing against .
looking at your match history.
you are silver 5.
the players you won against were bronze 3.
so yes, it makes perfect sense that you should gain less Lp for that win. as you are playing at a bronze 3 level, not a silver 4 level.
if you want to normalize your LP gains and losses back to 15 in either direction you must first prove yourself better than bronze 3 by winning more than 50% of your games against bronze 3 players to the point where you are consistently matched against silver 5 players such as yourself.
the only reason you are still silver 5 is because you simply cant fall out of silver 5, its impossible. otherwise you would be currently in bronze 3 rank and you would be gaining and losing close to 20 in either direction.