If you look at the transfer bar in the diagram, i.e., part number 41, you will send the bump at the top of the transfer bar on the NMV. The OMV did not have this bump. Sometimes it is a little to thick and the hammer presses it directly against the frame and the trigger return spring does not have enough power to pull it free.
One way to see if this is happening is when the trigger is stuck to the rear, hold the gun sideways and look at the hammer. Then pull/push the trigger forward. If the hammer moves forward a few thousands of an inch, it was sitting on the transfer bar. If it does not move, you have other problems.