Take it apart & have a look.

If it's the same as my Scorpion mag, the spring loaded pellet carrier is slotted breach side & part of the breach side back plate is a moulded in bar that stops on the loaded pellet,
you can see how it all works, if you look in from the breach side & move the pellet carrier.

If it's missing the pellet, that bar is broken/worn away, You might be able to build it up again with epoxy, worth a go with the price of BSA mag's.