[MadMax]

Does the GBB fwoof all the gas out on the malfunctioning shot? If so, check the reed valve inside the nozzle. If this is not preloaded forwards (it should poke out the nozzle a bit) then something has gone wrong with the return spring or reed.

If not:

Check the mag valve lock on the mag and the valve disconnector tab on the left side of the frame (poking out the top under the slide on the left side).

The Magna blowback system uses the valve lock on the mag to lock the valve open after the firing pin bashes in the valve stem. The valve is locked open until the bb leaves the breech and the reed valve in the nozzle changes state powering the blowback internals to cycle the slide.

If the mag valve does not lock open, either the valve latch is siezed down (cannot lock valve open), or the firing pin isn't pushing forward far enough to lock the valve open.

Have him try firing on a mag cooled in the fridge for 3min. If the gun fires fine on a cold mag then the hammer spring may not be pushing on the firing pin far enough to lock the valve open.