Well. IMO. You should try out new champs. I hit a plateau every once in a while.
Real example from me:
I hit 30, play ranked, play random champs, placed in bronze 1.
I play Nunu, did pretty well with him, started maining him, got to silver. Like, 20-2.
I start doing bad with Nunu, I start maining bliz, I got to gold.
I start doing bad with Bliz, I start playing Janna and Malphite. Got to plat.
I start doing bad with Malphite, so I started only playing Janna, got to Diamond.
I'm pretty decent with Janna, so I keep playing her. But then I found out that I'm also a pretty good Shen. So I started playing Shen. Got to Diamond 1.
Tl:dr: Find the champ that is just right for you.