Are you saying the pistol did not fire in DA 3 times, but most of the time it fired in SA? Did the hammer fall, but the pistol failed to fire while the safety/decocker was in the upright position? Were you getting primer strikes on those rounds that failed to fire? Did you break it down, clean and lube the pistol before taking it to the range? It's definately not a break in thing. I would call Beretta, or seek advice from a competent gunsmith.

I've had the PX4 full size bought new for over a year now it's never failed to fire. I can release the slide lock on a new 17 round mag and it will fire. It does not matter if I use the lever or rack the slide it goes bang. I also have a sub compact 4 I bought used its never failed to fire.

If it was the Federal it could be that from a recalled batch,was there a pin hit on the dud round?My other guess is the round either was too long or it didn't totally chamber leaving the slide shy of battery just enough to cause the firing pin block to not dissengage or reconnect the trigger.I don't know these internally so I'm guessing on that one.

where the rounds fully chambered before you tried to fire?

one of the problems with using the slide release to load a round from a full mag is the slide doesn't always have enough force and momentum to chamber that first round. thats why a LOT of instructors will teach you to rack the slide off the stop and not use the slide release it's self. it gives you that little extra space and spring tension that can mean the difference between a chambered round and a failure to chamber. I don't think this is an FTF issue

I'm wondering if he had the magazine fully seated? Or was running it new out of the box and it failed to go into battery chambering using the slide release(although unlikely). As Tio noted I've never had any issue chambering a round w/ the slide release on my storms, the slide's all hit home hard in my pistols and I've never had any type of FTF issue.