Icicles or Frost in Freezer.
Poor cooling can be caused by a heavy frost build-up on the evaporator coils. A panel can be removed to expose the coils on the inside of the freezer. If there is a lot of frost built-up inside the freezer then you've got three things to check: 1) is the freezer door closing all the way? Is something preventing it from closing? 2) is the gasket seal broken? Even a hairline crack in it will allow moist air to enter and freeze into ice. 3) something may be wrong with the self-defrost system. Refrigerators are supposed to self-defrost approximately every 6 hours. A defroster is a heater that melts and ice build-up and sends the resulting water down into the drip pan. If the heater element fails then frost will build-up on the evaporator coils and your refrigerator's efficiency will begin to diminish.
To troubleshoot your self-defroster you can remove all of the perishable food from the refrigerator and freezer, turn the thermostat to off inside the refrigerator or unplug it, and leave both the freezer and refrigerator door open for 24 to 48 hours. You may have to prop the doors open with something, but try to get them open all the way. Even if you see the ice inside is melted it doesn't mean the water and ice on the evaporator coils is gone, so let it sit. Have plenty of towels ready becaue you may overflow the drip-pan. After this is done and it's dried out, turn the thermostat back to normal and see if things get better. If not, you may have a bad defrost timer, a bad heating element, or a bad defrost thermostat (aka bi-metal switch)..
In summary check the door first. Your door may not be closing all the way. Check to make sure your freezer door isn't being prevented from closing with bags or boxes of food.
A seal around the door could be cracked. If the seal isn't absolutely perfect then ice will form in the freezer. It has to be 100% sealed.
Defroster not working properly.