The Nemesis works by dumping all the air for one shot. There is a sprung loaded device that opens the valve by pulling down on the valve when the trigger is pulled. The valve stem can make its way loose, and the flange end approaches the end stop, so when the trigger is pulled the valve stem barely moves and little or no air comes out. Just make sure the valve stem is tight.
Another common fault is the barrel making its way slightly forward in the clamps, then the breech doesn't seal tight against the transfer port.
The first time you take a side plate off the grip, be aware that a loose pin between the two halves carries one end of the main spring. When the cover plate comes off, you lose support for one end of this pin, and either the spring or the pin - or both - can ping out and go and hide under the furniture.