iCloud photo sharing stops after a while

Hello,

I am trying to share 669 photos to a single album via iPhoto in my mac. This is the process I followed -

I imported the entire album in my iPhoto. Selected the 669 photos, clicked on iCloud photo sharing, added the pictures to a new album. Then, the pictures started uploading. I could see that from the small pie in the top left of iCloud which got completely filled and read "uploading 669 photos" or something similar to this.

Unfortunately, after adding about 10-15 photos to the shared album, the remaining pictures never got added to the album. I tried the process again after a while, the same thing happened. iCloud added about 20 photos and then did not upload anything. Why? Any help would be appreciated.

Thank you..