How can I fix my upvc patio doors?

The normal opening door has dropped at the top and will not close. I found an adjustment at the bottom of the hinge using an allen key which will lift the door to allow it to close. Unfortunately what ever is happening to the door happens again and the door drops. As a short term expediant I can keep using the allen key but can anyone tell me how to fix this permanently.
The doors were made by Magnet and are 6 years old.

Barrie Shield
December 2005