I, too, am having trouble with the nextpage command in my blog posts. However, I reinstalled the WP default theme and its not working in there either, so I'm thinking it must be in the wp_link_pages code:
<?php wp_link_pages(array('before' => '<p>Pages: ', 'after' => '</p>', 'next_or_number' => 'number')); ?>
That line resides inside div class="post".
Anyone see anything in that wp_link_pages line that doesn't look right?
Thanks!