Make sure your students are logging into Woot Math via the student dashboard. https://www.wootmath.com/student/login
Your students should see all of their assignments after they login. Their dashboard should look something like this:
If your new assignment is listed on this page, next make sure that the student(s) in question have been assignment the new assignment. Since it is possible to make an assignment only to individual students, sometimes not all students will have the same assignments. To check this, click on the down arrow next to the assignment name:
In this case, we see that 8 students have not been given that assignment yet.