Current Subscribers dropdown menu returns "No items found"

When I try to manage my Subscribers via the Subscribers page, despite the fact that I can see the subscriber count (ie. Category One (15) ) most of the categories return a result of “No items found.” when I select the category and click “Filter.”

The odd thing is that the problem is not consistent. A few categories will consistently return the correct results, other categories will occasionally return the correct results, and a few will never retrun results.

I’ve made certain that I’m on the proper tab (Registered Subscribers vs. Public Sunscribers).

Any idea what might be causing this particular issue?

http://ww.wp.xz.cn/plugins/subscribe2/