how to share all photos to iCloud stream

As long as you haven't exceeded any of the hourly and daily limits or the other limitation in this Apple document, My Photo Stream and iCloud Photo Sharing limits, you should be able to upload all 970 photos. Have you tried adding the additional 20 separately?

What format are the photos that will not upload? Perhaps PSD files, edited in Photoshop?

iCloud Photo Sharing supports JPEG, TIFF, PNG, and most RAW photo formats

Can you edit the photos in iPhoto, that amercing from your shared stream? Try to find out, which photos are missing and try to find a common feature.

Videos added to a shared (photo stream) album can only be 5 min or less in length. If yours are longer, perhaps that's why it's failing to upload.

I'm not sure if it would help or not, but you could try going to System Preferences>iCloud, click Options next to Photos, uncheck iCloud Photo Sharing, restart your Mac, then go back and check iCloud Photo Sharing again. Since it is consistently failing to upload 20 photos, I'm not optimistic this will help but you could always give it a try. Otherwise I think léonie is right; you're going to somehow have to locate at lease a few of the missing photos in order to see if they have common attributes that are preventing them from uploading.

I suppose another thing you could try is to select all the photos, export them as .tiff files (to be sure they are all converted), then import these .tiffs to iPhoto and try adding these to your shared stream. This would test whether converting the format would be enough to resolve whatever the issue is with the missing 20 photos that is preventing them from uploading.