When I program in a flightplan manually by using airways and intersections/waypoints, it looks fine when I'm on the ground. As soon as I take off, the first leg of my flightplan dissapears, and soon, the entire airway section is gone. What I end up with is the last waypoint before my destination. It's like the FMC continuously degrades the plottet route. I think it has to do with airways. 

However, I notice other people flying long distances, multi-hour trips. How do they make it stick? I hope not by plotting individual waypoints. On a realistic flightplan that would take forever... 

Are you entering in the FMC or from the flight planning screen?

In the MCDU in the cockpit 🙂