Next Page Links Don’t Work for Categories

I have customized a WordPress install for a client based on the “default” theme that comes with WordPress.

The site is on an IIS server and this has caused some anomalies in the past, but here is the one we are currently trying to resolve:

At the bottom of the blog main page there is a “Next Page” link that goes to…

http://www.Website.com/index.php/page/2

That’s fine. That’s what it’s supposed to do. However, if we go to a top level category page and then go to that same “Next Page” link at the bottom of the page it should go to…

http://www.Website.com/index.php/category/CategoryName/page/2

But it doesn’t. It still goes to…
http://www.Website.com/index.php/page/2

The code that generates the “Next Page” link on the blog main page as well as on the category pages is this…

<?php if(function_exists(‘wp_pagenavi’)): ?> <?php wp_pagenavi(); ?><?php else : ?><?php posts_nav_link(); ?><?php endif; ?>

I’ve been pulling my hair out over this and can’t figure it out.