Often during turn-arounds in the 737, the cabin crew cannot be bothered to open all of the 180 plus overhead air vents - most of which have been closed by passengers on the previous leg.
Also despite Boeing advice in the Supplementary Procedures section of the Flight crew operations manual of how to cool the cabin on the ground, the cabin crew do not close the window shades on the sunny side. This means the air-conditioning packs are often unable to efficiently distribute their cold air throughout the aeroplane because of closed vents. Nothing wrong with the air conditioning systems in the Boeing - its usually human error or sheer laziness.