Skoda Octavia defrost failure

On some cars there is a humidity sensor which when you switch onto demist screen it activates air con , if needed , to clear faster . Don't know if this is what is happening with your car .Had it on a few cars and know of a few friends who get annoyed with it when they notice air con on and switch same off .If you have a cabin filter , check that also as if needing changed can restrict air flow .Check engine temp o.k and not running cooler due to sticking thermostat which will affect heater ability to clear screen .