iCloud Photo Library "stuck" on one device

I hope the attached diagram helps explain:

I have multiple devices all using the same AppleID and all connected to iCloud. I have enabled the iCloud Photo Library, and my main computer (the orange Mac mini above) is set to retain originals, while the other devices are set to optimize storage.

Of the 30,000 or so photos on the Mac mini, about 27,000 have successfully made it to iCloud and sync with the other devices. I can add photos from a digital camera to the Mac mini's library, and they do eventually make it to the cloud and the other devices. The Mac mini however exhibits two troubling behaviours:

1. Photos taken with say, the iPhone, go upto the cloud and get picked-up by all of the other devices EXCEPT the Mac mini, and
2. The remaining 3,000 or so photos are "stuck" and don't go up to the cloud.

I have rebuilt the Photos library twice. I have disabled and re-enabled the iCloud Photo Library function on the Mac mini several times. It goes thru it's updating/sync process, only to ALWAYS get hung-up at the same numnber remaining. EVERYTIME.

I have let the process go for months quite literally (since the availability of the iCloud Photo Library feature), and have not progressed.

HELP!