That's it, sometimes for no known reason, sometimes I roll a ranged attack with 2 less points than my ranged attack bonus. Whether the target is moving or not, and whatever her size is. That was a few days ago and I didn't check recently if the problem subsits tough, so I will look into it again.
Not a bug. Moving targets give -2 ab on ranged attacks even the slightest shuffle counts. You also receive a penalty to AB if your target is too close and you don't have point blank shot.