next_posts_link not displaying on search results

In the building of a template, my search results page (search.php) refuses to generate a next_posts_link.
In an attempt to trouble-shoot, I am using the same template as for category.php, for which next_posts_link displays normally.
Category page:
http://beta.gurumagazine.org/category/askaguru/
Example of search results page:
http://beta.gurumagazine.org/index.php?s=guru

– note that the next posts link is formatted to appear as a down-arrow.
– note also that the layout of the search results page looks odd (doesn’t display ‘search results for’ at the top) because I am using the same template as category.php to work out why it isn’t working.

Can anyone suggest why next_posts_link displays in category.php but not in search.php?