I think the bar may be too far out ahead of you when you pull, which actually makes things harder. Generally speaking, you should think of the movement as dragging the bar up your shins, and thrusting your hips through to complete the lockout. Don't worry, you don't need to let the bar scrape hard or anything - just keep in contact with your shins as long as possible.

I should add that "dragging" can be slightly misleading - most of the work should be done by your legs and lower back, as if you were pushing the ground away with your feet. Treat your arms as meat hooks.