The most common cause of this is that the magazine doesn't sit quite right in the gun. Normally this is due to a screw on the underside of the gun not being tightened correctly. Please refer to the attached photo.

With the magazine OUT, hold the magazine release button in, and tighten the screw to the left until the button doesn't drop. At that point, release the button and back off the screw SLOWLY until the button pops. You should now find that the magazine seats correctly and the BBs feed normally.