The reason could be that the drawing from which you are copying your entities has been closed.
— Make sure the file from which you are copying your entities is still open.
— If it is closed: open the source file, select the entities to be copied using the “Copy” or “Copy with base point” commands from the Edit menu
— Open or switch to the destination drawing keeping the source drawing open, go to the Edit menu and choose the “Paste” or “Paste to original coordinate” commands.
NOTE
The right way to copy is always by using the “Copy” command from the Edit menu.
