Well there is only one peice inside the gearbox that can cause the trigger to not move.
in front of the trigger the little physical safety that stops the forward mortion on the trigger. If that bit is installed wrong the trigger may not move on semi.
ill link a pic of how it should look but it should be flat on top and tapered on bottom.
not my pic, credit to the owner. But its the bit in front of the trigger.
only other thing would be selector plate but it goes from safe to auto so likely not that.