Students contact you declaring that they're unable to see the Test / Discussion Board / Item / file in your course.
Solution 1: Can They Find It?
Is the content easily accessible in the course? Is the link on the Course Menu visible to users? If not, you'll want to make it so and communicate to your students where in your course the content can be located.
Solution 2: Check Dates
Make sure that the item that the student is looking for has no "Date Restrictions" in it's settings. If it does and isn't supposed to, simply uncheck the box to the left of the "Disply after / until" option.
Solution 3: Adaptive Release?
Have you set Adaptive Release rules for the item? If so, has the student met all the criteria necessary to view it? You can use the Performance Dashboard to determine how far the student has come in fulfilling the criteria.