refrigerator help?

[WS6]

Ok I have a refrigerator with the freezer on the bottom. Well, a couple weeks ago the coils int he back of the freezer iced up and the frig on top got warm. I unplugged everything and defrosted that coil. I also cleaned the heat exchanger coils under the frig. Plugged everything back in after leaving it for a day and it's worked great. This morning though it was a little warmer. I opened the freezer and the coil at the back has iced up again. Why?

Last time we checked the compressor and the freon level. Everything was fine. The freezer door doesn't necessarily shut all the way sometimes. It didn't shut all the way a few days before the top stopped working the first time. I figured that caused humidity to get into the freezer and that created the ice. This time though, the door has been shut and was not left open at all. So how can the ice be forming?

Thanks