[Danke]

Is this a winding magazine or a low or high cap?

Sometimes if the mag hangs too low the feed tube won't hit the button and release the BBs. You'll get the first hit from when you slam in it but nothing more unless you tap it up again.

For this you just need to trim a bit off the catch so it feeds when it's hung it the lowest point.

Highcaps will avoid this because they'll force the ammo up, but no one here wants to encourage you to use those.