Attempting to upload jpg, png, mp4 files from Windows to iCloud via iCloud. Receiving "Excluded (not synced)" on 10% of files (90% successful)

Receiving "Excluded (not synced)" on a number (actually 10%) of the files in "iCloud Photos" folder on Windows 10 machine. I've looked at all the relevant help on this site (and elsewhere) and still find no valid answers. iCloud Photos folder on Windows contains 19,161 files and only 1,914 have the "Excluded (not synced)" - the remainder of the files show either two statuses: "only stored in iCloud" or

"file has been downloaded to your computer". Neither the folder nor are any of its contents shared.

• I have 200GB of storage with 144GB free - so my storage plan is more than adequate.
• The fully qualified name is not the problem since many, many of the files that successfully uploaded have a longer filename than these that fail (many are simply "IMG_####.jpg" and the file types are basic: e.g. jpg, png, mp4. I have no subfolders within the Windows "iCloud Photos" folder ... it contains 19,161 files.
• I have rebooted - nothing changes. I have deleted files from the "iCloud Photos" folder and moved them back into the folder; yet still the status shows a status of "Excluded (not synced)"

Any suggestions and or helpful links would be most helpful!