Could be the result of a few different things. First, use your glutes at the top of the pull - that will finish the hip extension. Second, make sure you're staying over the bar long enough - if you open up early and the bar hits/drags low on the thigh, it will often prevent you from finishing the extension. Third, make sure you're properly balanced - if you're too far forward or too far backward, you won't be able to extend completely (one because you're falling forward and have to rush get under the bar, and the other because if you extend all the way you'll fall backward).
Video would help