Thermador simmer burner won't light

The rear left burner will not light. It started to be erratic at first, now it sparks but the gas valve does not open. Do you think it is the simmer control, the Pot, the gas solenoid or the gas valve?
I also notice that when the front two burners are on max the left burner does not seem to burn as intense as the right burner. Any ideas?

Very weird, I replaced the oven ignighter because the oven was sputtering, it not only fixed the oven BUT my left rear burner started working. You got me, but it has worked for the last two days.