Another option is, you installed the trigger and the pins were too tight. You have driven the pins in hard, and the force of this has deformed the two side plates of the trigger, interfering with the free movement of the parts inside.
Be certain the pins have not smashed one side toward the other and pinched the triggers lower sear in place. You can test this with your thumb or finger on the sear as you operate the trigger. If it does not return to the raised position very easily, that may be your trouble.
I also had some other issue cause this in the past and at the moment, I can't remember what it was. I've seen interference with the stock cause trigger issues. I've seen bolt issues cause a firing pin not to fall when a trigger was improperly installed. Look for hidden mechanical interference.
Ah, also, crushing the trigger may be causing the trigger to never cock as well. For the life of me I can't remember what it was that caused this to happen for me before, but I remember it was an easy fix... Sorry to be vague.