Hmm. That is unusual. When AutoCAD opens a drawing, it creates a lock file in the same folder as the drawing. For example, if your drawing is called "drawing.dwg", a hidden file called "drawing.dwl" will be created in the same folder. If another user tries to open the same file, the .dwl file lets AutoCAD know that it is already being edited.
Somehow on your system, this process is breaking down. I'm not quite sure what to suggest.