I have noticed that if you click through to a unit from the curriculum link the associated forum is visible but if a student takes a course and click start course and goes through the courses process there is no associated foum link?
I have selected forum association in both course and unit?
I checked one of your course named "first steps & essential tasks", in this course I opened your second unit "backup wordpress automatically" from both the course curriculum and from the start course. Here are the screenshots of both and in both I can see the forum attached.
Screenshot 1 from the curriculum:
http://prntscr.com/9rl3yk
Screenshot 2 from unit inside the start course button:
http://prntscr.com/9rl4e7