Can you post the model number please.
Also is this a fan assist oven?
Ok that's great. When you turn it on does the little red light come on?
Also is the fan inside the oven running?
Ok so is the fan ok then?
Did you test the motor at the back for resistance?
Sorry my fault when you said you tested the fan to make sure I thought you ment you actually remove the back and tested it.
If the little red light is coming on and it does get warm but not hot enough then you are looking at a faulty fan motor here. As on these the fan has to be running to circulate the air around otherwise if not you get the fault you have now.
So as the red light is coming on then the thermostat is working so it wont be that and as it does get warm the heater is coming on so this just leaves the fan motor so as its not running this will be why its not working correctly.
To cure this you will need to get the fan replaced and that will cure the fault.
If you need anymore info just get back to me.