Adobe won't let me save over documents after editing.

I create large agenda packages and often have to go in and edit. When I try to save the edited version it prompts me with that file already exists do you want to save over it, I say yes and then I get this message "The file may be read-only, or another user may have it open. Please save the document with a different name or in a different folder". I am the only person in the office who works on these documents so I know that is not the issue. My work around is to save with the same name_1 and then go back later, delete the original and change the name on the edited version. Any ideas?