sometimes whenever you get a mission that takes you to an NPC initially to receive the mission instructions, it doesn't show the guy to go talk to. so you have to cancel the mission and retry taking it. I was having problems with it most recently with the construction missions. I did notice however, that one of the npcs that usually are the one that it has you talk to said that he couldn't give me any further missions without finishing the one I was on, which was none other than the new one, not the one I had just finished. this is on city merc missions, but I have also noticed it for some other factions that I cant currently remember