Hotpoint FF4DK Frost build up in freezer

Hi All
Hope you could give me some advice

I have a hotpoint FF4DK Quadrio fridge/freezer and there is a buildup of ice at the bottom
of the freezer section ,and not allowing cold air to circulate to the fridge section .
When turned off and defrosted it will work fine for about 1 week and then there is the ice build up again .
could this be caused by a faulty defrost heater or electronic damper thermostat


Thanks in advance
Mike

Is the door seal in good repair?