Simmer burners won't light

I have a Thermador Professional Range, Model PRDS304 with simmer burners on the left and regular burners on the right, grill in the center. Suddenly both simmer burners do not have gas flowing to the burner and will not light. The electric starter works and the right burners ignite normally. When either of the simmer burner control knobs are turned, the electric igniter sparks but it’s obvious there is no gas flowing to either burner. I have cleaned both star burners and checked the gas opening visible below the burner cap but the problems seems elsewhere. Any ideas?