stock GI type trigger?
to me it sounds like a dissconnector problem. it might be sticking in the trigger housing or it could just be "wrong".
i had one recently that did not have the hole for the dissconecter notched down far enough in the trigger , so i had to open it up enough to get the proper action with the hammer.
i would check the hammer-trigger and dissconnector fit/action to see what is going on there.