Sometimes, that happens. We can whip out something that looks amazing, and then the next one just doesn't feel as good. Keep going, though--each time you draw something, whether you like the end result or not, builds your technique/style, and over time, your technique will become more consistent. :)
What Voluptua said. Plus, you're looking at your next efforts with different eyes... now you have a better idea of what you can do, and how that might line up with what you want to do, so you might be a bit more critical. But don't think of it as criticism, but critique... what could be changed? Is there a better process? On what aspects should I focus more? That sort of thing. =)