The site has WP ShowPosts with the premium addon installed.
There are currently 4 categories defined: Events, Front, News, and Resources.
When I try to add a new category to WP ShowPosts, the only categories that are available are News and Resources.
Why aren’t all the categories available?
I just found the answer. I didn’t have any posts with the Events or Front categories, just News and Resources. After adding the Events and Front categories to a several posts, they became available to WP ShowPosts.