If it was a regular, graded discussion then there is nothing I can think of that could cause that behavior if the students replied, Tammy. Even in a graded GROUP discussion the instructor should still be able to see all individual student replies in SpeedGrader. There is not any issue that I am aware of regarding this, and in fact I tried it again just now on a sample course site just to be sure. That's not to say that it's working for everyone, though. I would definitely file a case with Instructure on this, then. But first, just a few added precautions:
-Eliminate browser issues. (Make sure the instructor is on an updated browser. Speaking of which...can YOU see the student replies in SpeedGrader....if you have administrative rights? If so, perhaps there's a browser issue.)
-If you have admin rights, you may want to see if you can replicate the issue yourself...perhaps on a test class or something. If yes, there's something odd going on. If not, then it may be specific to your instructor.
Sorry I can't think of more!