Are you able to download the presentations and run them in native PowerPoint?
I have found over the years that the web-based readers are sometimes problematic. I try to avoid using browser-based readers for MS-Office documents (regardless of the Office program) whenever possible.
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Brian -Windows 10 Pro, 64-Bit, Version 2004, Build 19041
The purpose of education is not to validate ignorance but to overcome it. ~ Lawrence Krauss