I have just been to a DG supplier/repairer and asked the same question as I have the same problem.
The guy said to open the door, push up the handle so the locking hooks come out and liberally apply vaseline to them. Then pull the handle down to retract them and ensure the slots are well lubricated too. Work the handle up & down a few times, remove excess vaseline.
Haven't got to my own yet as I'm fixing my patio doors, but it's next on the list. Good Luck. Steve
Steve, May 2010