I have had this happen many times in both worn and not so worn guns when reassembled. There is a small rounded nipple at the end of the coupler that sets in the rotary bolt and binds in the machined portion of the rotary bolt in which it rides and rotates. I would completely disassemble this and reassemble it making sure the coupler isn't upside down as well as loosely assemble the coupler to the top lever making sure it is somewhat loose as for the nipple not bind in the rotary bolt. More than likely when assembled or messed with during cleaning it has not fell fully in the worn area in which it rides in the rotary bolt causing it to bind. It takes very little force with metal on metal friction to bind one up. I have also had to tap with a plastic end hammer to move the top lever which unfreezes it but do not recommend this.