Hi there notreallytristan!
Thank you for posting in Apple Support Communities. It sounds like you are having trouble syncing photos from your PC onto iCloud Photos. We'll be glad to help you find a solution for this issue.
To start, do you have a large photo library? If so, we'd like to check to see how much storage space you have available on iCloud and if it can manage the additional photos. To check, use the steps here: Manage your iCloud storage
Keep in mind, If you exceed your iCloud storage limit, photos and videos won't upload to iCloud and your library won’t stay up to date across your devices.
We hope this helps.
Cheers!