Permalink Postname Not Working

I am running WordPress 1.5.1.3, and I have permalinks enabled…categories work just fine and so does my rss feed. The only thing that doesn’t work is if I click on the title of one of my posts, it says “Sorry, no posts matched your criteria.”

Why would it be that categories work fine and list all the posts in that category, and my rss feed works, but postname links don’t work?

This is my permalink structure: /index.php/%monthnum%/%day%/%category%/%postname%/

mod_rewrite is enabled, but for some reason I cannot get it to work without doing /index.php/ first….because when I do that, my RSS feed doesn’t work without doing the wp-rss2.php thing.

I have tried deleting .htaccess and putting in a blank one, I’ve tried it with a variety chmod’d settings…but wordpress is able to edit .htaccess just fine, so I’m not seeing how that’s the problem?

any help would be very much appreciated, thanks.