Q: iCloud sync issues between devices..

My iMac has the Photos app, pix integrated fine from iPhoto.

But, my photos will not sync and go into iCloud drive/photos on any device.

I have 20,000+ photos. I do have an Amazon Cloud for pix, but it won't take them all, either.

iMac, iPad, 2 iPhone 6's..

All particulars are done. Correct buttons in proper position.

My iMac is a 2009 model. Is there an issue with iMac being older and not syncing properly with iCloud?

Thank you,

TBH!!