Trusses may appear to disappear from layout when the the trusses are not engineered and the background color is set to light gray. This happens because the color for un-engineered trusses is also light gray so the trusses will blend in with the background as shown here:
To resolve you will need to go into the User Preferences\Colors Tab to the Back ground color and select a different color. If you still would like to use gray just select the darker gray.
Now with the background color set to dark gray and the un-engineered trusses set to light gray they can now be seen on layout: