Why would beer in a keg go flat?

Once it’s no longer full it will start to go flat as the gases in the beer are released into the air inside the keg. The only way to stop that happening is to re-fill it with beer, or as you say, put it under pressure. Do you have an air-compressor?

Congratulations though. Blue Moon is an absolutely delicious beer. One of the bars I go to in Manchester sells it. If you can’t find a way to re-pressurise the keg, I vote for drinking it all. Any excuse. :)

Beer goes flat because the CO2 disappates into the atmospheric air that enters the keg as the beer flows out. We had a CO2 system for our kegs in college. It kept the beer from going flat.