syncing photos on Mac with iCloud

Check, if there are items in your Photos Library that are unable to upload to iCloud as described here:

Check the status of your library in Photos for macOS - Apple Support

Are any items shown as "Unable to upload" or "on this Mac only"? Such items have to be removed from your library and converted to a more compatible format. Also check all videos in your library, if they still can be edited on your current system version.

Photos will only download items, once it managed to upload all items from your Photos Library into iCloud and merge them into the existing library. A single problematic item that cannot be synced with iCloud can hold up the syncing.

Since the discontinuation of My Photo Stream on July 20, the only cloud photo synchronization tool that Apple offers is iCloud Photos, which is fully bidirectional (omnidirectional if you include an iPad and/or other device(s)).

The iMac will need to complete its initial upload phase before it starts downloading. Is the number to be uploaded changing? Do you have enough free storage in iCloud to handle the iMac's photos?

There is one draw-back when using Shared Albums. They are cannot transfer your photos in the full resolution and with all metadata.

Photos you add to a shared album will be downsized to 2048 pixel at the longest edge and some metadata will be removed. They are into show your photos to others, but not for transferring photos to archive them on a Mac.

See the paragraph "Supported file types for Shared Albums" in Shared Album limits - Apple Support

"Shared Albums upload a copy of your data. Downloaded content may not contain the same information as the original.

• • Image formats: HEIF, JPEG, RAW*, PNG, GIF, TIFF
  • Video file types and formats: HEVC, MP4, QuickTime, H.264, MPEG-4 

Shared Albums also support special formats that you capture with your iPhone, like slo-mo, time-lapse, Live Photos, and Memory videos. When shared, photos are reduced to 2048 pixels on the long edge, except panoramic photos, which can be up to 5400 pixels wide. "

I am having the same problem! And the "me too" response is just circling.

Help!