ClassiPress doesn't display custom categories in post_class

Hi! I am using ClassiPress to create a directory site and I want to style ads differently based on their category, the problems is after I have added the post_class function to the theme, the out-put doesn’t include the category name or id (either wold work for me)

I have searched all over for days and experimented with ton’s of ideas for fixes and nothing is working.

Post_class out-puts the following:

post-24 ad_listing type-ad_listing status-publish hentry

but no categories.

Any ideas on how I can get the categories into the post_class, I thought this was default.

ClassiPress uses custom post types and taxonomies would that have an effect on the output and if so, how can I overcome that?

Thanks a bunch!

Jeremy