This issue is usually something simple. Assuming you have not solved it, check your menu item for the category where the description is not showing. Sometimes, you'll have the menu item set to override the category. This will also happen when you have that menu item manage (or link to) multiple categories, upon which the menu item then controls parameter output.
Another overlooked issue, check to see whether that category is inheriting category output from another category.
If you have not solved your issue, please provide links and additional description.