A slight possibility exists that the interior of the lower rcvr which houses the fire control group (hammer / trigger / safety) is out of spec.

I had that happen on an AR into which I was installing an aftermarket trigger module assembly (Ra-140). I remedied the problem by removing material (approx 1-1 1/2mm) from the sides (about the forward-most 1/3) ( the top edges) and the top edge of the front of the "cassette". Worked great afterward. In my case the trigger would not reset due to the assembly being bound-up within the rcvr. If the assembly is installed to tightly which might be binding the hammer, that could lead to your problem.

I would remove upper from lower and basically do a function check.

Protecting the rcvr where the hammer will impact (or walking hammer forward), cock the hammer and pull trigger. If it "fires" go to next step.

After having pulled trigger and hammer goes forward, keep the trigger pulled back. Cock hammer again and release trigger. The trigger should reset. If it reset correctly, pull trigger again (protect front of rcvr against hammer impact). Hammer should fall as if striking firing pin.

This next test isn't mine but it's good. Disregard any reference to photos. Given that yours is an assembly module, I assume it's assembled correctly though the function principles are the same as a mil-spec trigger.

"It definitely sounds like an FCG problem. My guess the trigger is not returning on release. As long as the trigger is held to the rear, the semi-auto sear (1) engages the hammer (2) when it's cocked to keep it from following the BCG when it returns to battery. This sear will not disengage until the trigger moves forward to it's reset position.

If the trigger is to the rear and the hammer cocked, engaging the safety will push the trigger forward and the semi-auto sear will release the hammer to be engaged by the trigger sear (4) which will release the hammer to fire the weapon when the trigger is depressed. You can test this by seperating the upper from the lower with the hammer in the released position, depress the trigger and hold it while cocking the hammer. Keep the trigger depressed and engage the safety. You will feel the trigger being pressed forward to it's reset as the safety pivots into position. In normal operation, spring pressure will press the trigger forward to it's reset. If the springs are not correctly installed (4&5), or of they are broken or worn, the trigger may not return".

Me again: If it malfunctions doing these tests I'd say most likely it's too tight. Also make sure the safety is atop the rear of the trigger / hammer assembly. Safety switch should be installed with hammer in cocked position.

If the trigger module has set screws and / or you're using aftermarket anti-walk/rotate pins, make sure they're not too tight. That can cause your problem. Snug is all you want.