Dear Rob T,
This is because the Windows Clipboard needs to use different formats (called mime types) for different kinds of clipboard information:
The "proxy" (application) that you are trying to paste into does not support direct clipboard data mime type from progeCAD.
More info: progeCAD "drawings" differ from "Paint" drawings in that it contains vector data (real geometry, coordinate data, etc), as opposed to raster data (pixels). The program that you are trying to paste into may accept a screen shot rather than a direct copy-paste from progeCAD.
Hope that helps.
progeCAD support, tips and troubleshooting forum. progeCAD works very similar to some versions of AutoCAD. Moderated.