It has come to our attention that there is a potential issue that arises in SpeedGrader for group assignments when instructors try to provide comments to students who are not assigned to a specific group. Instructor comments appear to not be saved in SpeedGrader. After pressing the Submit button, the comment disappears. The comment DOES show up if you look at the student's Grades page Submission Details for the assignment in question. So, we can (hopefully) assume that the student is seeing the comment.
Canvas is aware of the problem, but be aware when grading group assignments that have been completed by individuals not assigned to a group. You may want to email those comments to students separately.