Multiple materials and assignment description require the student app to upgrade to iTunes U 3.0. From the help (What’s new?):
Some new course features require iTunes U 3.0. For example, students who do not upgrade cannot view assignment descriptions and cannot access private discussions, homework hand-in, or multiple attachments per assignment. In order for your students to have the best experience, make sure your students upgrade from iTunes U 2.x to 3.0 before you include any of the new course features.
In addition, based on some other threads, ensure the iOS device is running iOS 8.3 or 8.4.