One thing to check; make sure the fan ( visible in the rear wall of the oven) is starting when you turn on. This can sometimes stick due to grease clogging from cooking fumes. A quick, CAREFUL flick with an insulated screwdriver will usually start it.
The full solution is to remove it and clean it thoroughly. It is accessed easily once the rear outer panel of the unit is removed (after disconnecting the electricity supply obviously).
Steve.
April 2009