In the online course that I am teaching, each unit assignment has a rubric attached. It is mind boggling to me that students submit work that doesn't include everything as outlined in the rubric. When the unit begins, I send an attachment of the assignment/rubric. It is also clearly displayed on the unit page. In the synchronous seminar held for each unit (student choice to attend), I also go over the assignment/rubric. For some, nothing seems to work.
I like that you had your class determine a rubric for the discussion grading. Although we are given a rubric to use (by the curriculum developers), it is not as straight forward as the assignment rubrics. The attributes are grouped and then given points. Discussions are the least favorite for me to score as a result. I love giving and using rubrics, and I keep trying to create one for discussions that is more objective and less subjective.