I recently found a problem with my link_pages failing to display the links to the additional pages. I searched the forums here and found no answer so to help benefit others this is what I found;
Most of us add whitespace, as force of habit, when adding "comments", however, when applying the <!--nextpage--> tag in code view to a page in order to break it into seperate pages, adding whitespace causes the function list_pages() to fail. You need to ensure that there is no white space in the tag and in doing so will correct the problem.
This issue occurred whilst using WPv2.2.
Hope that it helps. :)