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The reason could be that the drawing from which are copied entities was closed.
-- Make sure the file from which they were copied entities is still open.
-- If it proves closed: open the file of the drawing of origin, select entities to be copied from the Edit menu and choose "Copy" or "Copy with point basis."
-- Open or move in the drawing of destination without closing the drawing of origin, Edit menu and choose Paste "or" Paste in Coordinates original ".
The right way to copy is always use the command "Copy" from the Edit menu