Two main things - get your elbows up a just a little bit in yOur rack position and push your shoulders forward and slightly up. You may already be trying to do this and just be limited by inflexibility - if so, keep working for it. You just need to get your shoulders supporting the bar and your elbows slightly in front of it.
More importantly, you need to stick with the leg drive longer before you transition under the bar. You can try doing a push press before the power jerk to help this.