So me and my buddy are doing the campaign on legendary together. We were half way through mission 3 but I had to go for a bit, so we continued from our checkpoint later on that night and once we completed that mission, neither of us go the achievement and it said the mission wasn't complete even though all the checkpoints had been unlocked. The same again happened on mission 4. We got to the end of the mission and his Internet cut out, so we re-done the last checkpoint and it happened again!! Has anybody else experienced this issue?
