MMR, how does it work?

So I am B1 on my role top, S4 jungle, and S2 mid. I mainly play mid...

Anyway...

So recently I have been noticing the higher I start ranking up, the lesser LP I am receiving. I am not quite sure how to make of it. So my question is, why am I receiving at most 15 LP points per win?

I know it's not much but still... it's demotivating how you spend so much time in the game and you only get 15... my W/L ratio is positive except my other roles... I am confused with how the mmr works.