This is a PSA gun I got as a Kit  a couple years back.  It is the first time I fired it and was using 223 Federal 55gr rounds.     It loads and fires fine on first shot.   Try for #2  and I get nothing and it won't go on safety.   Pull charging handle and out comes round number 2 where it had been fully seated .  The primer also had a healthy dent in it.       I  checked then opened up the receiver to see the hammer was down.     I check how the hammer functions, locking back, dropping and it seems fine.     I put it back together and it does it again each time the exact same way.    I installed my buddies upper and bolt assy then tried it to see the same thing.     So whatever is going on must be in my lower.   We put both lowers side by side and it looks as though everything is identical in the trigger group , nothing installed out of place by me like I was expecting.     I never saw or heard of this sort of thing happening when I was in the Army but then again I wasn't an armorer either so I left the innards alone as I was told.  

disconnector spring upside down

 

Do you have a heaver buffer than you need ?

Load one round, leave the empty mag in and fire.
Does the bolt lock back ?

Ensure that your hammer pin is firmly seated through both sides of the receiver.  If you installed your spring upside down it will not lock the pin in place, allowing it to walk out.  This will allow your hammer spur to not engage the disconnector, causing it to follow the bolt forward.

Thanks.     No it has no mods whatsoever and I did swap the buffer with the other gun to no effect.       The upside down spring theory looks to make the most sense.   For some reason tiny spring have always been a headache for me so putting it upside down would make sense as long as it would still go into place is possible.    And you say if backwards the hammer would appear to function normally , lock back, release ect?  
  Looking at the depth of those primer dents I'm surprised I didn't "runaway gun " right through the whole mag.  I don't suppose there is any way to be able to verify that springs orientation without taking it all apart is there, that's not my usual luck.

Well, I can't get a pic to post on here. anyway it  does look similar to your pic only larger. .      That disconnector does seem to float freely with no forward pressure at all.    Did I forget a spring under it?     That's what it looks like to me. says Homer Simpson,   "Dohhhhhh"
I'm betting I never had one or somehow forgot it.

Ha, not until I get a spring.      If it's not there then I either didn't get it or it's long gone to places unknown.     So where do I get one of those?   For that matter I even lost the spring that keeps. the rear  reciever pin from coming all the way out..  Hell, I even lost the buffer retainer spring some place or another but a spring for a ball point pen cured that.      I am really getting an education here on things I used for many years and never gave a second thought to.