You aren't finishing the pull. It's almost like you're trying to go up with the bar and down win your body at the same time. It won't work unless you're superman. Watch the feet for the guy in this video and how he finishes the pull fully extended and on his toes. And watch yours.
A good lift is about getting a big pull and being fast to get under the bar. When you get extended and on your toes, the bar path changes just a little to move it over head (and behind in line with spine)
MDUSA does a lot of drills where the do their snatch pull and then their full snatch. Goal being the pull is identical in both lifts