Tasks that are set before a student joins a class will not be allocated to them. This is because students that join a class during the academic year from another school would not want to see all the tasks that were set previously that they have not had an opportunity to do.

After navigating to the tasks for a class by selecting the course, class and Tasks button, you can assign an existing task to students by clicking on the name of the task:

Then click on Assign:

The select the student/s you want to assign to the task:

Click Next then Finished to save your changes.