Gas to top rings shut off by glass lid?

Cannon Chester cooker (10540G) glass lid operated gas to rings cam out of sync. With lid closed gas still to rings, lid open no gas to rings but lid closed just about 10 degrees and gas to rings. Has the cam that pushes on the flat top of the gas valve moved/slipped slightly and if so how is it adjusted to allow the gas not to flow when lid closed and to flow when lid is fully open?