iCloud Backup

I recently updated to a new phone, and upgraded my iCloud plan to 50GB to ensure that I would have all my photos on my new phone. However, on the new phone I only have about 2000 out of 5000 photos, and the photos that synced seem to have been chosen at random. I can’t find a way to sync the photos that didn’t sync in the first place, is there a way to do this?