why is frost forming in my frost free freezer?

I have a Siemens 438 litre frost free fridge/freezer. It's just over 4yrs old and has recently started forming ice in the bottom of the freezer so much so that the freezer drawer gets stuck. I have to chip away at the ice (using a plastic ice scraper) at least once a week otherwise it's reaches the freezer door and starts melting then there is a pool of water on the kitchen floor. Have switched it off completely letting it 'warm up', cleaned everywhere, re-started it hoping I'd sorted it, but the ice keeps forming - why?