You need to make sure the NO ONE in your party has completed the Richtofen EE. This was the problem I was having.
3 of us were starting a game on Tranzit and wanted to ensure we had the Rich EE complete for Buried. We kept trying to get Rich to talk to Stu but it wouldn't happen. Turns out, one of our buddies had completed the Rich EE on Tranzit last, so that is what was stopping us from completing. My other buddy and I went and started up a custom game just the 2 of us and we heard Rich right away. Completed it on round 3.
You need to make sure that everyone has either not done an EE at all, or that they have completed the Maxis version and are now wanting to switch back. If even just 1 person in your group has completed the Rich EE, you will not be able to complete it a second time.
Happy Hunting.