Star rating is not very accurate, so you might have been having bad luck and playing harder low* maps. Even if that's not the case, there's not a LOT that you can do other than play osu.
Of course, there are some ways to optimize your learning while you play. Pay attention to EXACTLY what patterns you have trouble with, and practice maps that have those. Preferably find a map that is a bit easier than the ones you want to play, but still has the same kind of patterns that give you trouble. Don't grind plays on a map though, because when you do that, you start memorizing the map and that means your scores will look better but only on that map, not other maps with the same kind of pattern.
EDIT: Also abraker, at least you've gained a few ranks recently. Other than that one bump, my tank has just been declining the entire length of the graph.
