Possible that you didn't complete a mission correctly.. It is a known bug .. usually if you restart the campaign and finish the mission differently (your last mission) it may start working correctly.. also missions work off your Prestige Points, but from what you are saying, Prestige is not a problem... However you may want to try moving towards ISC space and see if the missions start triggering again.. also some of the missions won't have a must play on it, and if you skip a hex that generates a campaign mission, then the campaign breaks.. dunno why.