Carry out these usual checks:
- Make sure session_start(); is called before any sessions are being called. So a safe bet would be to put it at the beginning of your page, immediately after the opening <?php declaration before anything else. Also ensure there are no whitespaces/tabs before the opening <?php declaration.
- After the header redirect, end the current script using exit(); (Others have also suggested session_write_close(); and session_regenerate_id(true), you can try those as well, but I'd use exit();)
- Make sure cookies are enabled in the browser you are using to test it on.
- Ensure register_globals is off, you can check this on the php.ini file and also using phpinfo().
- Make sure you didn't delete or empty the session
- Make sure the key in your $_SESSION superglobal array is not overwritten anywhere
- Make sure you redirect to the same domain. So redirecting from a www.yourdomain.com to yourdomain.com doesn't carry the session forward.
- Make sure your file extension is .php (it happens!)
Now, these are the most common mistakes, but if they didn't do the trick, the problem is most likely to do with your hosting company. If everything works on localhost but not on your remote/testing server, then this is most likely the culprit.
Hope it helps!!!