The first pull in the snatch, meaning the dead lift from the floor to above the knees, is essential to a good snatch. The most common error i see in the box especially with heavy snatches is the athlete letting their hips rise before their shoulders. This can happen for multiple reasons, the athlete may lack hamstring and back strength, or might be rushing the pull and losing tension in the midline. However, if the first pull is off, the lifter may be out of position the rest of the lift.
Exercise: 3x10 snatch deadlifts to knees ( focus back tension)
Here is a great video from cross fit, really focus on his first pull, or his dead lift to his knees. I really like his emphasis on back tension and keeping his butt down on the first pull.