Hey mate, I recently had the same problems your getting. First of all, try holding down the heat button for about 10sec or until an sign 'FH' appears on the screen. It should overide all the sensors and push out as much hot air as it can. Make sure the engine is warm too. Give that a try and if it still doesn't work then I think it could be something to do with the motor which controls your hot/cold air.
Let me know.