I've heard some complaints with not getting a campaign achievement (this happened to me with 'mercy killer') and with not getting the expert achievement. After some digging around, I found out that you can fix this by just voting for a restart campaign once everyone is there, at the beginning. The problem was that some people would miss part of, or all of, the beginning movie. Which counts as you joining late and not being qualified for that achievement you want.
So rule of thumb? If your after something; restart the campaign as soon as everyone joins.