The Host may have disabled the ability to increase the memory limit.
To see if the change to the memory limit is taking effect, try using the following:
<?php
echo ini_get('memory_limit').'<br />';
ini_set('memory_limit', '256M');
echo ini_get('memory_limit');
?>
this will write the memory limit being used by the server before and after the ini set, if it is not changing, you should contact the host to see what options there are.