Possibly the teacher has set it that way. By default, students cannot view homework from home. There is a good reason for this: students should learn to manage their own personal planner. The teacher can use a homework presentation module to display homework in the classroom for all to see.
However, depending on the teacher’s settings, sick and absent students can automatically view homework from home in their dashboard for 5 days. They are not in school and therefore receive the information about what they have to do.
