how can i stop my freezer frosting up in the top two shelves ?

After defrosting my scandinova 6 ft high 2 door freezer and then switching it on again in a matter of a few days, the very top ledge and the next ledge down (which holds the uppermost door flap) both freeze up to such degree that i cant open the catches to drop the flap to slide the upper drawer out.I accept that freezers can eventually freeze up but the above happens in days. The temp guage shows -4.