search field only working on homepage

Hi
I have a search field in the header to search for users/members and for some reason it only works on the homepage, it seems to be blocked on all other pages, as in – you cant type anything in the field.
below is code

<div class="headersearchbox">
   <?php $results_page = site_url().'/search-results/'; ?>
<form name="search" method="post" action="<?php echo $results_page; ?>">
<input type="text" name="search_string" id="search_string" class="headersearch">
<input type="submit" name="submit" id="submit" value="Search" class="headersearchbutton">
</form>
</div>

so works here
http://photohunters.org/photohunters/
but not on any other page – eg.
http://photohunters.org/photohunters/portfolio/

i havent disabled it apart from media queries, and even when i take out all the media queries it still does not work
Can anyone advise? HELP?
Thank you
Melanie