iCloud Photo Library not uploading anymore
I tried Photo Library briefly and had a similar problem. The progress bar would go to about 40% then disappear. The problem for me was a lack of storage space, and so it would just fail at a certain point. Go to General > iCloud > Storage to check the amount of space available on your iCloud account, then check the amount your photos take up (I believe this can only be done in iTunes). If the photos don't fit in the available space either buy more space or just turn off the feature as it wastes battery if it is not actually being useful.