The latitude is less likely to matter than the time of the month. If it is at or near new moon, you will not see the moon at night.
The "wrong side of the plane" reason can be seen as a subset of the "small window" reason, so I would say it is a combination of the small window (combined with the plane's orientation) and the point in the lunar cycle at which you are flying.
To improve your chances of seeing the moon in the future, fly during or near the full moon, and, if you have an east-west flight, sit on the port (left) side of the plane in the Northern Hemisphere, or the starboard (right) side of the plane in the south. If you are flying from west to east, reverse that.
The full moon will be lower in the sky near the summer solstice (late June in the north; late December in the south). It will rise later and set earlier, too. If you try to see the full moon in the winter, near midnight, it is likely to be too high, unless you are very close to the pole.
If you are flying a north to south route near the full moon, the moon will be in the east (to your left) in the early evening, overhead near midnight, and to the west (right) as dawn approaches. Again, if you are flying from south to north, reverse that.
To see a dramatic crescent moon, fly a few days after the new moon on a route where you can look west just after sunset. Or, fly a few days before the new moon on a route where you can look east just before sunrise.