I've noticed this with a lot of pieces of mine...I'll have a general idea or concept down, and once I get the gist of it, I struggle with what else I think my work needs. I look at a piece and say, "It needs more detail, but how?" So I add something that could add that would make it more dynamic, and it doesn't seem to make a difference. Or, I'm working on something with a lot of detail, like right now, I'm drawing the backyard near my house, which has woods with a lot of leaves, and in trying to draw out every single leaf, I get frustrated, and think "Isn't there an easier way to do this?", or "Why can't I get the shape right of all these leaves?!"
Maybe It's just impatience, maybe dissatisfaction in skill. I'm not sure why.