Recently after grading several discussion posts, I noticed some grades showed in the Grade center and some did not. Upon further investigation, the grades not showing in the Grade center did not even indicate from the student's cell intersecting with the item column an attempt was ever made.
For some reason, I believe I read in Dr. C maybe 2 years ago, this could occur if a student saved their post as a draft before submitting it. Unfortunately, i can't remember how to remedy this anomaly.
I would think asking the students to repost without saving the posting as a draft should work. If there is something the instructor can do without creating more work for the student, that would be the preferred solution.