How come when I try to play the campaign on gears of war it says it doesn't read it but versus works???????

The disk is most likely damaged or scratched, this is what happened with my friends, try getting a washcloth and gently rub it with water and if that doesn't work then go ok eBay or Gamestop anywere Tay would sell it try to find a disk cleener so u can recover your disk then it might work