STAR M249 Para - Box mag troubles

[SneakyNinja]

I have STAR M249 Para, which has been great fun to play with...but the BBs are no longer feeding, if i disconnect the battery and pull trigger the auto wind works, but when the battery is connected and the box attached, it will fire the few BBS that got into hop, but then just dry fires.

Am i right in thinking this is more of an electrical fault in the box mag, if both feeding and firing can't be done at the same time. Is this fixable or do i need a new box mag?

Thanks for your time