The most likely cause of the problem is a failure of the regulator that controls the affected ring.
Obtaining and fitting a new regulator should cure the problem.
Obviously, you won't know 100% for sure until you take the top off and check the wiring but, there's a small chance that you have a burned out wiring fault.
It's very uncommon whereas the regulator is a common fault with various failures.
Check it out, see what you find and get back to me If you need more help.
If you are unable to take the appliance apart yourself, sorry, you'll need to get someone in to do it for you.
Hope this helps