When you say that Photos "doesn't download all the photos from iCloud" do you mean that it downloads some photos but not all photos, or that it downloads no photos at all? Those would be quite distinct problems.
If it is not downloading any photos at all, start by checking that you are signed in with the same Apple ID and that you have enabled iCloud Photos in both Photos > Settings > iCloud and Settings > Apple ID > iCloud > Photos.
If it is downloading some photos but not all, can you categorize which photos download and which do not? Recent versus not recent? Shared versus not shared? What status is shown at the bottom of the All Photos view?