BTM, besides all the other possibilities mentioned, the most obvious is the way you are resting the rifle on the bags. With sporter weight rifles and hard kicking calibers, its not easy to hold the rifle exactly the same from one session to another. Try moving the spot on the forearm where the bag is contacting it. The amount of downward pressure you exert will also cause a slight shift in zero. These things are much easier to determine than sending a scope back.