A very common complaint, and of course each person’s ideal air temperature is different so it is impossible to please everyone. Here is some background information.
Although the outside temperature at altitude is bitterly cold, the cabin air is taken from one of the stages of the jet engines (before combustion so it is still pure uncontaminated air) and at the point it is extracted it is at 200-250 degrees C. This is called bleed air. To make it usable in the cabin it has to be passed through airconditioning systems to reduce the temperature. This consumes energy, and in turn the engines use more fuel in producing the energy. So the airline itself always likes a warm cabin – because they spend less on fuel.
The cabin crew on shorter flights like a cool cabin, because they’re moving about a lot and don’t want to end up sweaty. On longer flights they prefer a warm cabin, because for much of the flight in between meal services they aren’t moving about much, and passengers who have had something to eat and drink are much more likely to go to sleep in a warm cabin and avoid pressing the call buttons,Apart from a couple of crew members patrolling the aisles, the rest of the crew can gather in the galleys and have a good chat, or take official rest periods. It is frequently alleged that crew members use the controls to raise the cabin temperature between meal services so they get fewer service requests.
The trouble is that even after all these years the design of airflow through aircraft cabins is not a precise science and there are persistent hot and cold spots on most designs. In addition stripping out panels to access fans, clean filters etc can only be done by ground engineers and is the sort of thing that gets deferred to the next major service check rather than being done in turnround between flights.