Focus on driving the hips forward to lock out the deadlift.
50% of deadlift videos I watch, I see guys leaving their hips back close to lockout. When you leave your hips back, your shoulders hang over the bar. This can lead to "knee bending".
Bending the knees is generally a subconscious action. It allows you to move your shoulders back where they should be for good leverage (think leaning back), but is poor form, lacks power and not the way things are done.
Test this out. Pretend like you're holding a bar right above knee level and then bend your knees, keeping the bar at the same spot on the legs. Now lean back slightly. See how this slight knee bend allows you to move your shoulders back a bit? It is merely a misguided attempt to get your shoulders back behind the bar, and will fail more times than not because your hips are not driving lockout.
The hips must drive the lockout. When you stall close to lockout, you shouldn't be thinking about pulling the bar up. You need to be driving your hips forward and shoulders back with all your might and will.
The next time you bend your knees analyze the video. You will see the forward hip drive stalling (or most likely non-existent to begin with) and the shoulders not moving back behind the bar. Once the knees bend the shoulders and torso will move back, but the deadlifter has lost his drive and power.
You must concentrate on exploding your hips forward on lockout and stop thinking about pulling.
You can see weak hip drive even with some very strong deadlifters. Layne Norton is one good example. He is leaving his hips back on lifts and gets a little hitch at the top as he seeks to find some method of leverage to get his shoulders back and "pull" the bar up. You will also notice he has fits locking out his lifts. Once Layne improves his hip drive his pulling ability will be scary.
Another example. No hip drive so he bends the knees to try and get the shoulders back and find some leverage. This is a textbook example, and shows where the hitch comes from.
Bottom line...drive your hips. Explode.