OK, so I think we've figured it out. The values of the seeds aren't changing according to when the set opens, as I previously thought. Evidently it's related to scripts that we're running to assign homework sets to students. Any homework set that is assigned manually (using the webwork interface) gives random values of the seeds. Any homework set that is assigned using a script appears to have the problem of repeated values of seeds for an individual student on a given assignment.
Sorry for the posting -- but thanks for letting me know that it's not a webwork phenomenon. Now to look more closely at our scripts...
-Patti
WeBWorK Problems
How are problem seeds generated?
This forum has a limit to the number of forum postings you can make in a given time period - this is currently set at 10 posting(s) in 1 day