Duplicate video files not working

Random video files don't work when transferred or duplicated.

One process;

I transfer .mov files from one computer to the local drive on the mac. Most of the files work- others just show the quicktime icon, but fail to open citing "The document “XXXX.mov” could not be opened. The movie's file format isn't recognized."

Another process;

I already have the video files (.mov, .mp4, etc.) already on the local mac- most files have no issues, but when I duplicate the file some just show the quicktime icon and fail opening with 'The document “XXXXX.xxx” could not be opened. The movie's file format isn't recognized."

One thing is clear- if a file fails, once transferred or duplicated, that file will ALWAYS fail if re-transferring or re-duplicating.