I see what happened... The zombie doesn't drop the item that is required to trigger the completion.
I am fixing this right away. Thanks for letting me know.
ok! I will try again later!
Cool I have fixed the bug :) And tested it properly as well this time :p
I'll send you a cheat code in your inbox if you like. It will give you a gun and some ammo, which you normally get at the end of the 3rd battle ground.
As a kind thank you for your feedback.
Ps. I don't see a way to DM you here, but I noticed you follow me on facebook, I'll send you the cheat code there.