I've tried changing things in the .css and in the theme html file like the header and so on.. but the changes don't appear.

I've tried logging in and out.

I've tried setting the 'Update module template .html files from themes/your theme/templates directory?' to 'yes' in the settings.

I've got no cache enabled..

I've deleted all files in the template_c/ directory that had anything to do with the template..

What other reasons can there possibly be?

I found an extra reason... it always helps to edit the theme.html file thats actually placed in your theme directory....

DUH!

I've done that before as well... except in my case I was looking at the wrong ftp account

Glad you sorted your problem.