Not getting credit for Completing mission 4

So i beat the 4th mission on coop and didn’t get the achievment for it. When i went to the campaign menu i had the 5th mission unlocked, but it said i didn’t have the 4th mission (infinity) completed on any difficulty. I defeated the rest of the campaign, and didn’t get any of the campaign completed achievments. I went back to the mission and finished it just from rally point charlie ( i had all the rally points unlocked, also) and didn’t get it. I don’t want to do the entire mission over again if it is not going to do anything, because i don’t get how it would since i have all the rally points already unlocked.