Windows icloud - some photos not synced at random

Hi, I'm having an issue with icloud for windows where half the photos won't sync. It seems to be completely random which ones won't sync, there are a mix of dates, file sizes and file types.

I wanted to get my old photos from 2012-2015 onto my new phone now that I have more storage capacity, so I copied these photos into the "icloud photos" folder on windows. There were a lot of files so I left my computer on for 3 days and nights making sure it had all caught up. Now icloud appears to have been finished for the last day or so, it just says "updated 1 minute ago" rather than "downloading x photos".

When I check on icloud.com, photos are missing at random. I can't seem to get them to sync no matter what I do. My current settings are "always keep on this device" in windows. What's strange is some photos say sync pending but are in my icloud, whereas other have a green tick but aren't in icloud.

There are approx 47000 photos in my "icloud photos" folder and 34000 in both icloud.com and my phone (my phone seems to be syncing fine with icloud.com). I can't seem to get the number to increas whatever I do.

Thanks