Subscribe to the forum by logging into the forum as that particular user. You can do this from "My Settings" or from the bottom of the thread view for the particular forum.

I seem to be having the same problem. What I found is it's seems to be link to groups. If I select a group to subscribe to in My Settings it does not get added to the Forum_TrackedForums table when I press the Update link. If I drill down into each group and select each forum it does get added when I update My Settings. I have not taken the time to look thru the code to see why the groups are not working.

Ron