this only happened to me twice, i don't know if its a glitch or a bug attack from hackers, i be playing a little bit of survival and all of a sudden i couldn't melee attack, i even tryed switching controls where melee attack is on square, but still ain't working, then i switched controllers, and still didn't work so i know the problem is not my controllers, its the game that Activision or
Sledgehammer needs to fix, its really hard to attack them when ur out of ammo, and unable to punch