You need to check that the fan underneath is blowing, and hasn't "frozen up", or as my neighbors had, a mouse got jammed in it.
Find the defrost timer, it will have a knob on the front of it, a wheel or something, you can mark to see if, after a while, if it is turning or not. There is a defrost thermostat in the freezer. It stops the defrost heater after the freezer temp reaches about 70F in the coils (from the heater doing its job), and will not let the heater again come on until the temp in the freezer coils has dropped below about 25 degrees. I had both the timer go bad and the defrost thermostat burn up., they are both cheap on the internet. Some defrost heaters are "glass" and fail easily, may have to pull trim and find the wires to the heater element and disconnect it and connect a plug and cord to it and plug it in to to make sure it heats up. Just plug it in long enough to determine its heating, and no more. Make sure the fan inside the freezer is running also.
Charles