Documentation:WeBWorK/How to Properly Assign Homeworks
To ensure the homeworks get assigned to all students, please make it visible first and then assign it to all students. Otherwise, the students who join the course between the time your assigning and the time you making the homework visible will not get assigned to that homework.
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