We have recently started using Vectorworks 2010 and are integrating viewports referencing external files and are having some issues. They seem to disappear for no reason. The viewport is there and can be selected, but nothing is showing up in them. Sometimes it happens if the referenced file is open, other times the referenced file is closed and it still happens. A new viewport can be added referencing the same file and show up fine. All layers and classes are turned on and show/snap/modify is on. Please help?