Sometimes when you try resuming downloads, it says-
Cannot resume downloading the file:It's possible that the address of the file is not valid anymore or your session has expired.Please start the download of the same file from your browser manually, and IDM will try to capture a new address or new session data to resume this download
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Generally what happens is that the address of the file on server changes and it tries to resume from the web page address(the highlighted address below is the changed one. Usually the address is with the same extension name as that of the file you are downloading.)
Press OK and then resume it.