iCloud Photo Library not Downloading/uploading

I have two photo libraries on my iMac (happened during transition to Photos while also moving library to different hard drive). To "combine" them, I uploaded one to my iCloud library at the introduction of Photos, and then switched libraries and made the 2nd library my default Photos library, and selected in System Prefs/iCloud to again enable iCloud Photo Library. What should happen here is that the new iMac library is uploaded to the iCloud library, and joins the pics previously uploaded for former iMac library. And then eventually it should download the entire iCloud library onto my iMac. INSTEAD... It identifies correctly how many photos should be uploaded to the iCloud library, but doesn't upload them, nor will it download the photos in the iCloud library. It's like it knows what it's supposed to do, but can't or won't do it. Help?