Usually when something stops it is due to one part rubbing against another part. The pivot end should rest against the cap jewel when the watch is in the up or down position but here should also be clearance between the pivot and cap jewel so it floats when in the horizontal position. If the pivot is too short to contact the cap jewel the cone portion will seat into the jewel causing friction. This could mean that the pivot is too short or the jewel and cap jewels are too far out when the watch is in that position. If the jewels cannot be reset than you may have to machine the cone of the staff back a little.