I've run into this issue a few times while creating tubing assemblies that consist of several different tube parts.
While in sketch mode editing the tube route, the tubing run shows up, as seen below.
As soon as I exit the sketch, however, one piece of the tube inexplicably disappears. I've tried deleting and re-making the connection points, redrawing the sketch, and everything else I could think of. Any idea how to solve this problem?