In case anyone else is having this problem, here is what I found.
We have a weekly live webcast that we stream through YouTube. I was downloading the videos in MP4 format from YouTube after live webcast completion. It was these videos that would not play on the Rokus. The one video that did play was one we pre-recorded with our streaming system. I have run all of the MP4s that I downloaded from YouTube through an application called HandBrake (
https://handbrake.fr/) and re-uploaded the "converted" MP4s and they play perfectly.
The only thing I can surmise is YouTube is doing something in the encoding that prevents the videos from playing on the Roku.
I hope this helps anyone having this problem.