Why are my photos not syncing?

Yes, provided all devices are logged in under the same Apple ID.

Learn more about My Photo Stream at https://support.apple.com/en-us/HT201317

And, more about using iCloud photos at https://support.apple.com/en-us/HT204264

And the difference between both at https://support.apple.com/en-us/HT205743

I also have this problem, Photos are syncing between my iOS devices but not down to my iMac.

A couple of weeks ago I turned off syncing on my iMac and turned it back on and all my images downloaded. All was OK.

Now, my Photos have not synced down to my iMac since 4th September.

How can I address this, any help appreciated?