iCloud Photo Library won't sync all photos

I've enabled iCloud Photo Library on my Macs & iOS devices... and have noticed that all my photos are not being sync'ed to iCloud... and the ones in iCloud are not being sync'ed to all my devices. (I'm running Mac OS X 10.10.3 on the Macs and iOS 8.3 on the iPad/iPhone.)

In my master Library on my iMac, I have 20,233 Photos and 344 Videos. According to the Photos app, all have been uploaded to iCloud.

However, according to iCloud - only about half of those photos are uploaded.

(I have plenty of storage on iCloud... I have paid for 200GB and am only using about 40GB.)

On my Macbook - I started the Photos app with an empty photo library, and then turned on iCloud Photo Library. On that computer, it reports a different number of photos - 9,923 photos. And, it too is supposedly "updated completely"

When I browse the Photos on an iOS device, it's about the same as the MacBook - but not exactly.

Anyone have any ideas here? I would expect all the photos from the iMac to show up on all devices.

Is there a log file somewhere I can examine to understand what is being uploaded and what isn't?

Thanks!