How can I fix my Canon Winchester cooker?

I have a Canon Winchester gas cooker, approx 15 years old, the pressure on the two larger rings, front left and back right both light (using a gas lighter not the ignition) but no pressure so not able to use, when turned on to highest setting they look as though they are on the lowest possible setting. The two other, smaller rings are also low pressure but can just be used. The high level grill and oven both light and work fine.....please advice as I love the cooker and don't want to have to replace it.
I have been told this is due to 'dirty gas', is this possible and how can I rectify?