Hi William, we are are aware of some cases as you have described where lines will seem to drop out… I will try to explain the issue the best I can. It actually is a SketchUp rendering issue that is most easily seen in LayOut but can be reproduced in SketchUp by exporting the same scene to pdf. This is a result of rendering the line work incorrectly.
This seems to occur most often with Sections when a SketchUp viewport is not fully shown in LayOut and stretching the viewport so that the entire section is shown usually fixes issue. However in your case it not help.
Investigating further I noticed that if I shrunk the Viewport I could get some of the missing lines to show up if I intersected the model with the viewport. This is forcing a re-render of the SketchUp line work producing the missing lines.
Did you model this in SketchUp or was it imported from another software?
Trent