Fridge section of Fridge freezer not chilling correctly?

I have a Samsung American style Fridge freezer model RS20CCSV. the freezer side seems to be ok but if I open the door and I am in and out a lot or after about every 6/8 weeks the temp light starts flashing to show that the temp is incorrect. The fridge is obviously to hot but it never goes back down to temp. I have found that I have to remove the covers from the freezer side and defrost the elements and within 12hrs all is ok. The fan works ok and spins freely. Any ideas what causes this.