I solved the problem with the page, the correct answer is ptriek's comment below.
Well I'm having a bit of an odd problem here, wp_nav_menu works on all pages EXCEPT my category pages (probably my archive page too but there's no links on the website to the archive pages so that's alright).
Although there is archive.php in my theme files, this controls what is displayed on category, tag, archive, author, etc. pages.
I just don't understand why, since every page uses the same header.php to display wp_nav_menu, it won't show up for the category pages.
Am I overlooking something? I've been banging my head against my keyboard trying to figure this out and it just won't work!
Using a very simple code:
<?php wp_nav_menu('container_class=menu-header&theme_location=primary'); ?>