There are two or three reasons to have a ftf other than poor quallity of a primer. Shot in the primer seating pocket can move the anvil. Picking up damp hulls and throwing them in a plastic bag. What Z said about spraying your primer tray. Getting spent primers mixed with good primers *I don't think this is to likely but not impossible for the very talented."
Now my story.
Several years ago the club I shot at had a corporate event. Closed for the afternoon, cook out, shoot all you want, you get the picture. Close to dark Rain set in and shut the shooting down. Two of my good friends were the first through the next day and it was as if it had rained once fired AA's. The owner let them pick up the emptys. Sun was out and they looked dry. I got to shoot the first round with them and the new hulls they had scored. The drop out rate was. by my quess. 20 to 25 out of fifty. The hulls were still wet inside. For what its worth.