Iphoto/Photo not downloading all pictures to iCloud

I have recently purchased a new MacBook Pro as an upgrade from my old Mac purchased in 2012.

I previously used iPhotos to store all my pictures before linking photos and iPhoto together so my photos are all in one place in my photo library; going from 2012 to current day photo streams. For some reason however iCloud is only downloading photos up to 2015, this is the same on all of my devices apart from my old MacBook where I originally linked iPhotos and photos together. It is strange as it only goes as far back to the exact same photo from 2015 and doesn't download any before this even though they are on the photo library on my old Mac.

I want to reset my old Mac to pass onto my parents, however I cannot do this at the moment as iCloud does not seem to want to transfer all my photos to iCloud. Could anyone give me advice on making sure that iCloud downloads all of my photos, not just the ones from 2015-current day?

Thanks in advance!