The data in the $_GET array is passed in from an outside call - but not always - as they may, instead be passed via $_POST. If you send them one way and ask for them the other, they'll not be found. Easiest solution: use $_REQUEST, which will read both types (also good if you change your method of passing data at some time, such as if it gets really large or have greater security needs).
That's an easy first try - if you're actually sending the data!
If you're not sure about that, see: THIS