Pull the bullets, push out the primers and do the "hammer" test on the primers that FTF. Before you smack them, look to see if they are burned. (turned black)
It's been my experience that MOST FTFs are headspace issues. (not a physical issue with the chamber headspace) Brass too short for the chamber, you pull the trigger,round gets pushed forward by the firing pin. You get what looks like a good strike but just not hard enough. Short rounds will also let the primers back out then get pushed back in making it hard to tell exactly what happened. That's where the "hammer" test comes in. Sometimes it's the QC of the ammo. Most are a little on the short (SAAMI) side. I roll all my own ammo so when it happens to me, it'd MY fault. Short ammo or contaminated primers.