what sort of community do you leverage for their knowledge? how frequently do you expose yourself to new ideas? do you practice abstraction in places other than code? do you practice effectively and take notes that you review? keep a journal? meditate? are you in good cardiovascular shape?
all of these are the tip of the iceberg when it comes to contributing factors. a plateau is a sign of stagnant processes.
I apologize in advance for what I'm sure is a frustrating tone. I'm sympathetic to your situation.