Grouse,
The tolerances between the breach plug and the block that holds the firing pin all go in the wrong direction causes that in about one in 200 Encores as far as I know the best answer is to shim the block out, it is an easy fix. The other possibility is the primer pocket is just to big. Some times using Winchester 209 for shot shell primers helps as they are the longest primer. Mostly the combination of the tolerances of the breach plug the barrel position and the firing pin block all go in the same direction on the same gun it just adds up to enough so that the primer is not held in place and so it blows back. Lee