I have experienced the same as Dan. When an aircraft on departure is approaching the line of the Polar area, it will disappear (naturally), but does not appear on the map as a Network flight (allowing for the delay of course), unless I also do what Dan does as well, ie, uncheck the "Process Hardware Flights" and then check it again. If I don't do that then they will sometimes reappear but not until a good 30 minutes has passed.
I first noticed this recently on a couple of occasions when tracking the departing A380 from Heathrow. Once it left my Polar area on it's route ((just pass Lambourne), I did not track it again as a Network Flight until it reached the Netherlands.