I suspect the "Rerandomize" button on the Library Browser page is broken. When a student is assigned a set (on the Homework Editor page) the are assigned an individual seed for each problem. The seed plus the problem template file determine the version the problem that a student gets for their homework. Their version of the homework will not change until the seed is changed.
You can experiment with changing the seeds by clicking one of the "Edit this problem" links and changing the seed before choosing the
"View this problem" radio button.
This tutorial might help as a general introduction to managing a webwork course.