Board Thread:Skyrim/@comment-114.79.58.34-20130712151152/@comment-7153552-20130712162654

It happens after a certain point in the main quest, so where are you in that line from the base game? Also, they have to travel (the world is live, remember that) from the docks to whatever twon you are at. So much like the courier, sometimes you'll pass each other and they have to turn around. If you've gone through the main questline completely, I suggest picking a town (Whiterun is good as it's ventrally located) and just hang out there for a few days.