Everyone reaches a "plateau" at some point in their MOBA playing career, and to get past this plateau, it's not enough to merely play the game, but to really work at becoming a better player. This means reviewing the occasional replay, studying specific players and emulating them, reading up on in-depth guides like
ChaQ's Mid Guide or
e-Hug's Warding Guides, breaking out of comfort zones, practicing mechanics in bot games and generally approaching the game less like a hobby and more like work/study. And it's common for players to encounter multiple plateaus as they improve, each one requiring a new level of effort to surmount.
I've never seen Sheever play, but from her notoriety as the weak link in pro-stacks, it seems she fell into the same trap a lot of players who avoid solo-queue fall into. They rely too much on their better teammates and inflate their own MMR, resulting in a much tougher solo-queue experience, giving them more reason to avoid it. This cycle builds on itself until they wind up stifling their progression. The solution is either to play more solo-queue games, or to ask for carry/mid roles in these stacks. However, I get the impression Sheever doesn't really care that much about getting good enough to really play with the pros. There's nothing wrong with it, Sheever's job is being a personality, not being a great player.
