Photos stopped syncing with iCloud

What happened before the syncing stooped? Did you move your Photos library to a different drive?

I take it, you checked if the library on your MAcBook pro is enabled as your system photo library and as your iCloud Photo Library in the Phots Preferences?

Does the Photos Preferences panel > iCloud show a progress bar? What is the status message for the iCloud sync? And what message appears at the bottom of the Photos > Moments view?

Yes, a progress bar is shown, but it does not change.

It may be hanging without any visible progress for days, if your library is large. My Photos Library needed a full week for uploading (40 000 photos).

Try a smart album to identify items that cannot be uploaded to iCloud Photo Library:

File > New Smart album:

Is this album empty? If not, what kinds of photos or videos are in this album? What is the file format of these items.

I have a similar problem. 8000 out of my 15000 photos show up in this smart album "unable to upload" and I have no idea why.

There are three possible reasons:

1. Your photos or videos may not be stored in your Photos library but be referenced outside the library. That will happen, if you disable the Photos preference "Importing. Copy items into the Photos Library". These items cannot be uploaded to iCloud Photo library.
2. The originals of your photos may be missing completely or cannot be found. The most common reason is having used a cleaning application, that modifies directly your library, like CleanMyMAc.
3. Or the file format of the items is not compatible with iCloud Photo Library,because it cannot be converted. This can be some graphics format, BMP files PICT files, TIFFs with higher bitesize than 8bit, foreign video codecs. iPhoto or Aperture allowed to import these items, but the Photos.app at www.icloud.com cannot handle them.

• If you have referenced files in your library, you can turn them into managed files by selecting them and using the command "File > Consolidate". That will move them into your Photos library and they can upload.
• If you have items with an unsupported file format, convert the format. Export the item to convert it to a JPEG, then reimport it and delete the original version from your Photos library.

deleted the photo library from Photos, and started a new library linked to my iCloud Photo Library. It was a radical solution, but worked.

That is a very good solution. 🙂 Only be aware, that any faces albums or project will now be missing, because they are not stored in iCloud.

It will be like described here: Use Photos and iCloud Photo Library on multiple Mac computers - Apple Support

Have you checked, if the number of photos in your library is the same as before after the download from iCloud?

Do you have enough free storage on your system drive to hold all Photos?

How large is your library? my library with 40000 photos needed more than a week to download from iCloud.

It does not only depend on the speed of your internet, but also on the traffic at Apple's cloud servers.

Only the photos that I click on and open are truly download.

Then do just that. Select 1000 photos or so at a time from your moments and use the key combination ⌘R to rotate them, then ⌥⌘R to rotate them back. That should force the download. Continue 50 times.

Your Photos Library is behaving, as if the flag "download originals" were not set. You might try to start your Mac in Safe Mode to clear all system caches. Then restart normally again.

See: Mac OS X: Starting up in Safe Mode

Did you also try to force the download by selecting large groups of photos at once and rotating the photos back and forth with ⌘R and ⌥⌘R?