All my photos didn’t download from my iCloud

I just upgraded from an iPhone 6 to an iPhone XS. And I backed up my 6, to iCloud right before switching to the new phone. After two days I noticed that many of my photos from before this March 2020 are not on my phone. I logged into my iCloud and I see them all on there. But only half of my total photos are on my phone.

I have tried everything including logging out and back in of my iCloud and my photos have not been downloaded back to my device. Please how do I get them back into my phone ?