Check your Auto Prime first. They are famous for wearing the little link between the thumb pirce and the primer ram. This would mean that your primers aren't fully seated, so the firing pin fully seated the primer for you on the first shot attempt, then the second firing attempt was able to fire the primer. I've worn out a number of Auto-Primes, and won't buy them, or pretty much any Lee product any more. RCBS & Hornady make hand primers that will seemingly work forever.
You may also have a slightly loose chamber, coupled with factory brass that is right at the minimum as far as headspace goes. You may have also pushed the shsuspecoulder back a bit since you full-length sized the new brass before loading. Since most of your shots fired on the second attempt, I'm blaming the Auto-Prime first.
Actually, "FTF" is the abbreviation for "Failure To Feed", but we all know what you mean. I'm just channeling my inner "abbreviation Nazi".