Documentation:WeBWorK/How to Properly Assign Homeworks
To ensure homework sets get assigned to all students, please make them visible first and then assign them to all students. Otherwise, the students who join the course between the time you assigned the set and the time you made the homework visible will not get assigned to that set.
The work flow should look like this:
Compose Assignment (not visible) ---> Change Visibility to "Visible" when ready ---> Assign to All Student ---> Assignment Open
Note: The time between those steps can be any time as long as they are in correct order.
To assign a homework to students, please follow the instruction here: http://webwork.maa.org/wiki/A_day_in_the_life_of_a_WeBWorK_instructor#Adding_Students_to_a_Problem_Set