The FCG is not installed correctly. The hammer pin is secured by a spring (J or D) in the hammer and the trigger pin is secured by the hammer spring legs. Hammer pin has a groove in the center and the trigger pin at each end. Spring tension in these grooves is what keeps the pins from walking.
(update) I took the lower apart and saw that the spring was not catching the trigger pin correctly. I adjusted the pin/spring and This seemed to have remedied the issue. Thanks for the advice. I will skip the KNS pins if I don't have any further issue.
Yes, the problem with building them is you want to build more! I'm looking (again) at 300Blackout since I have two 5.56 rifles now. I initially wanted a pistol AR but I'm throwing that out the window until I find cheaper pistol barrels.
I've also come across many other parts that are not fitting properly. I have an ATI complete polymer lower with takedown pins that are super tight it becomes a pain installing the aluminum upper receiver on and off .