Excerpts not appearing

I want to use the excerpt function to leave brief descriptions about my blog posts. I am arranging my blog posts by categories and for some reason certain categories are not displaying excerpts while other categories are.

For instance, I have tried leaving excerpts for this page: http://www.noahdebonis.com/category/portfolio/commercial/promotional/

and for this page: http://www.noahdebonis.com/category/press/

but only the excerpts on the latter page are displaying. If anyone has any insight as to why this is I would be very grateful.