What do the primers look like in the casings that are empty. Are they flat? What kind of shells were they? Handloads? If so they could have been a bit hot or not hot enough. Out of battery usually occurs after a hang-fire or light primer strike that doesn’t immediately go off. People jump on working the action to quickly and pay for it just as quickly. I’ve seen an out of battery in 12 gauge. No one was hurt but it will wake you the hell up.