I think it's a classic memory leak-type bug. Each seat back screen probably ends up generating a new connection to the onboard map server every time a customer opens the map application. And when the map application is closed, it doesn't properly free the connection.

At a certain point in the flight, after enough people open and close the map, no new connections to the map server can be made and the application refuses to load.

If you want the in flight map the whole flight, the solution seems to be to open the map application and never close it, and watch movies on your laptop or tablet.