I had this issue too and noticed that nobody is getting a reply. Here's what I just did which reset my problem. I noticed in iCloud Settings that both my iPhone and my iPad (which were not sending photos to my iCloud Photos) had a message which read 'Updating 1 photo' but it would not clear. So instead of turning off and then back on (thus having to delete and redownload stuff to my devices) I tried this:
Just plug in your iPhone / iPad to the computer and do a manual import of the photos you currently have online. That imported the photos and also cleared the message. I also just did a test with a new photo I took and it synced fine.
Now the important thing will be to determine what caused the process to get stuck in the first place. I can think of two scenarios based on my setup but can't be clear: One is that I received a rather large video via whatsapp which was not synced and also I had plugged my iPhone directly to my computer over the weekend for it to charge and did not check when I unplugged it; maybe it was halfway through a sync?
Anyway, hope this works for you as well.