Vannette 7000 cooker with grill lighting problems?

Can you help please? I'm having problems with the grill.It lights ok but refuses to stay alight after i release the control knob no matter how long i hold it in.
I laid a mirror on the floor of the grill,and noticed that only the front of the element was alight.The thermocouple was on the back of the element.I lit the Gas at the back with a lighter.I then released the control knob and it stayed alight.This worked ok every time,as long as i never used the spark ignitor.So why does just the front of the element light??