Do you start them in the air? I found that if you start them on the ground and make their first action "Take Off" that usually helps. If you start them in the air they will be airborne but their engines won't be running and they will plunge or glide straight down. I think this was the case with cfs1 aircraft that I had transferred across to cfs2, i can't remember if it happened with stock or add-on aircraft for cfs2.