The top two layers in my table of contents are the most important. Streams and some point shapefiles. They overlay a raster. In layout mode I switched from portrait to landscape. Immediately the streams thinned so much they are barely visible and the points are so small they are invisible. (I know they are there because they blink when I turn the layer off and on). Correct locations though.
There were two similar topics online. One suggested checking the size and position in the properties to make sure it isn't negative. Done. The other was to delete the border. Done.
Unfortunately neither worked.
I'm on ArcMap 10.1 if that helps.