iCloud (iPad and iPhone) do not match FOTO app on iMac- pics are missing

I have a problem regarding the iCloud sync. Not all pictures from my iPad or iPhone will appear on iMac's Foto App. I have a really huge photo library.

I did the following for test reasons: I took three picture with mi iPhone 6. Within a short time they were visual on iCloud (Safari) and on my Foto app on my iPad.

On my iMac only one picture "arrived".

Also in the database on the iMac a lot of pictures were missing. I activated the iCloud on my mac. And reactivated it. I also contacted Apple support but until now the problem was not solved.