almost 2 years ago - Shurenai - Direct link
Cheat Mode, Creative Mode, or Debug Mode enabled, all disable achievements.. So if you have them enabled, that'd do it.

But for the achievement in question, It may be intended to be 'in one game'; In which case, until you've killed 869 zombies in your new save, it won't go up anymore; As it's already at 868, you need to reach and surpass that number to increment it again.