Does session variable saving work in general on your site?
Try setting a session variable on one page:
$_SESSION['test'] = "works";
and then write the value on another page:
If session variables aren't working in general, then this is probably an issue with the session.save_path setting in your php.ini, usually pointing to a non-existent folder or the folder doesn't have the correct permissions.
If session variable settings do work in general, then it must be an issue where the session variable is stored in the first place. I'd need to see a copy of the page that saves that value to see why it might not work.