by ben reynolds.
Have you done "view source" to see if the html for the link is there and just not showing? In erratic behavior, I always suspect that page hasn't finished loading and therefore the html has broken. That's because there is no way to turn off feedback files for one person and not another in the same assignment.
Another possibility might be to look at the filenames for the invisible files. Are they complete with no spaces or odd characters? Many servers do not like special characters in filenames.
And a third question. Can the graders see the files even though the students cannot?
And a fourth question. It isn't clear to me whether this happens for everybody in one assignment but for nobody in another assignment.