I have the same problem. My iPhone 5S is plugged in. iCloud is enabled (and paid for), and I have over 2 gigs free of iCloud storage. The bottom of my Photos app just says “94 items to upload,” and nothing happens. — Except if you watch closely, sometimes it will read, “Uploading 94 items,” but then it just flashes back to “94 items to upload.”
I will sync to my computer manually, but this was very disappointing and a bit worrisome.
I’m running iOS 8.4.1. I restarted my phone (by pushing and holding Home & Power) and this did not solve the problem. It has not uploaded any new photos in two full days.
The only thing I can think of is… two days ago, I briefly connected to a captive portal Wifi (and successfully authenticated). Probably has nothing to do with it, but who knows. It was working fine until recently, as far as I can tell.