GPS navigation not functioning?

Hi everybody! I have problems when I'm trying to fly flightplans with GPS, when I choose NAV and GPS some of the planes don't move at all. The altitude and speed is all correct, but the plane doesn't turn an inch. I should first say that this doesn't work on some of the planes I've installed, for instance a Meljet 777. Since there isn't a panel for the 777 in FSX, I use the default 747-400 instead. I've also tried some 777 panels from FS 2004, but they're also not working for different reasons. When I installed a 747-400 (Singapore Airlines, Project Opensky) and use the default 747-400 panel it works perfectly! I don't get this... Am I doing something wrong, or is there a bug or what's the problem? I've used the Meljet 777 thousands of times for GPS flying in FS 2004 without the slightest problem. Please guys, help me with this! By the way, after a week of crashes to the desktop, the sound all of a sudden disappearing and the FSX freezing, this is really getting annoying now... Otherwise, FSX is a true masterpiece.//Greetings from Sweden