Neff double oven cooling fan?

After re-reading my original question , I would like to amend it ( to be clearer)
When I use the top oven as a "grill /toaster" the fan should blow air through the "top" slot , this used to happen , but lately the air force has reduced to the point that it now is virtually zero.
But if I use tis top oven as a "oven" ( turn knob to different setting ) air with plenty of force blows through both slots.
Any ideas what is wrong?