Images and video stop appearing on Macintosh?

Using a iMac, The night before a service I prepared slides and videos. The next some slides don"t work. I reload them from the library. I import them again from the desktop. The don't appear in the service. There is no rhyme or reason. It can be a jpeg or an mp4 file. When we open the service on a PC laptop it works just fine. Any ideas?

Are the files being uploaded to Presenter on the Mac or the PC?
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