Consider what Matt said. Also, this issue seems to appear for people who are tight in the lats, triceps and the other stuff that attaches to the upper arm from underneath. During a press, those muscles can relax appropriately to allow you to move properly; during a jerk, they tighten up and prevent the shoulder from moving properly.
Work on stretching the pecs, lats, tris, etc. and do plenty of light pressing and pressing behind the neck as warm-up for your jerks.