Gahhhhhhhhhh

Can anyone tell me why my drawings that I don't even work on turn out way better than the drawings I put effort into? I mean I just start doodling in class and they turn out way better than the stuff that I take an hour to work on! It's so frustrating because if I scribble something I like and then try to replicate it, it turns out looking like crap.... Please explain this craziness to me?