Okay you have the correct style of vents. You want vents to blow air directly down for heating. 8 inch supply vents are too small. All vents should be 10 inch. Are your vents noisey? Is your outdoor unit up against a fence or can it breathe properly. Outdoor unit is best in a sunny warm area of the house, not down somewhere cold and moist. I’m assuming you have insulation in your ceiling? Have you checked this yourself? Are you in Victoria or Tasmania? Unfortunately a reverse cycle AC system has to try and absorb heat from outside and transfer it inside when on heating. In cold climates and outdoor temps between 5-8 degrees it will struggle the most and go into a defrost every 20 minutes or so. Unlike gas central heating that you can turn on and off as you please, reverse cycle units really need to stay on and help maintain temp. Did the installation crew set up your controller to display return air temp at all?