If the fan spins then air will move. Assuming the blades are correctly oriented, hot air will be extracted out of the power supply, otherwise air will be sucked in if the blades are the wrong way round. If the air movement is minimal then it just means that the fan is running slowly. That could be normal behaviour. If the fan speed is controlled according to temperature it would only speed up when the heat level inside the power supply reached a clip level.
One thing worth checking is the amount of dust/fluff clogging the fan blades. You can do a pretty good job of cleaning them with a string vacuum cleaner or one of those canned compressed air blower thingies.