I'm not sure about drawing, but I guess the same ever happened to me when I write. It usually happens when I forced or rushed my writing. And then by the half-way I'd feel, "Oh come on, where's my glory? Why is this felt so suck?"
Err, suggestion...? Um... Well, everyone handle their art differently, you know. Some people can work on their art anytime anywhere, while the other need the fitting mood and condition to work out something satisfyingly (that's me
). So, if it's me, I'll firstly spoil myself or wait until I felt damned then continue on. Or, you can keep trying and repeating until you get something satisfying.