Ads Subcategory never loads

I've encountered a strange situation with the Ads section.  I can create Ads subcategories just fine in the admin area, but no matter what I do, when attempting to actually POST an ad, I can select the MAIN category just fine, but the SUBCATEGORIES just sit there saying "Loading" in the selection box, but never does anything else.  You can even create an ad and when trying to post it, while the system recognizes that there is an ad, it doesn't display anywhere.