If you'd like to have students complete an assignment that you've already assigned to the class, you'll notice that the ASSIGN button is grayed out and there is a check the Assign box for the class.

From WORK, you see an image like this one.

To re-assign an incomplete assignment, you'll want to make sure that the assignment is Available and the Default.

From CLASSES, choose Assignments from the middle gray column.

Locate the assignment from the list and make sure that the slider is on for Available and the box is checked for Default so that students will have to complete the assignment.

Students who have not completed the assignment will be directed to the assignment.