Looks like it is because sometimes your current stage shows 0 delta-v remaining, even though there is fuel in the tank. When you plot your node, nothing moves when you drag your burn vectors. However if you derp around with adding/deleting stages you can get your delta-v to display properly, and your maneuver will display again. Hard to describe but it shows in this video. You can see the frustration but at about the 8 minute mark what you can try to resolve his issue. I'm sure this will be an easy fix for the devs. Loving the game! Cheers!
