Zanussi ZCG7690XN Hotplate problem?

I've been given this cooker by a relative but there seems to be a problem with the hotplate. The front & rear right hand side burners only ever appear to be on 'simmer' regardless of the heat setting. When you turn the knob the flames get slightly larger & smaller but not significantly. The other side works fine as does both ovens & grill. I've tried thoroughly cleaning the rings in case they were blocked but they're still the same. Any help would be greatly appreciated.