SMEG Fan Oven not heating up problem?

Can you please help diagnose the following problem and suggest solution please

I have a SMEG SUK61MFX Oven (Gas hob electric oven)

The oven is not heating up when the knob is turned to ventilated heating element and ventilation position - the fan is working

The lower oven is heating up with the knob turned to lower heating elemment and ventilation - the fan is working
The oven is heating up with knob turned to upper and lower heating element without the fan