Carefully touch this tube. If it is extremely hot and will boil spit off a finger, it's time to check refrigerant. All too often, the problem is a restricted expansion device or capillary tube. The difference here is a cool discharge line and cool condenser coupled with uneven frosting of the indoor coil. If this coil is frozen over, you have another type of problem.
Most coolers have no active defrost system.
If this is not happening the coil can freeze over. It may be sticking or set too low. This will cause the cooler to warm up inside and may be caused by low refrigerant charge.