If the forearm is free-floating when not on a sandbag, but touches when resting on the bag, I suggest opening up the channel in the forearm. It's too easy for you to sight in with the sandbag but have your support hand in a different location when in the field. It's possible to be a bit off for your point of aim/impact.
At any rate, I always set up at the bench with the front sandbag in the same location as where I place my support hand when in the field. Consistency.