When a student joins a class and that class already has assigned tasks then the student will receive any task that is:
(a) not past the due date, and
(b) has been assigned to the whole class up to that point
(a) For the teachers who love to have lots of overdue days allowed, we don't want students receiving 20 overdue tasks the second they join a class.
(b) If a custom task has been assigned to a smaller group of say 5 of the 30 students, it could be an extension task or remedial task. We don't want newcomers to be placed with that group by default.
So basically, if it's a whole class task not yet due then a new student will receive it, otherwise they won't.
Note: Tasks that are past the due date but not yet expired will not be assigned to newly joined students. Also, changing a past due date after students have already joined a class won't automatically reassign a task to these newly joined students as Mathspace makes the decision to assign tasks upon a student joining the class.
If there is a task that has not been assigned to a newly joined student due to the reasons above you can duplicate the task and reassign to students if you wish.