If your bottom freezer has ice build up, (you will see it frosted up), you do not have proper air flow. Check to see if your defrost elements are working. They are in the freezer compartment. in the evaporator. There is a defrost timer on the bottom of your fridge. You can turn the knob until the refergerator turns off, then listen in the frezzer compartment to se if the elements are working. (You will hear them sizzle)