Oh, Warrior I you tricky pose!! This pose can take a lot more focus than the rest. In my own practice and watching others, squaring up the hips is a tough one for many. For most really.
One of the most common "mistakes" is setting up legs for warrior II and then turning the upper body forward.
Take your time. Put a little space between your feet. Angle the back foot in and not to the side. And then read this article. Great tips!!