Photos - shared album / iCloud photo sharing

Hi,

Since upgrading to Yosemite and Photos I have been unable to add photos to a shared album on my iMac. I can create albums, invite subscribers, view the album on my iPhone and iPad but it just won't allow me to add photos. I have tried numerous times with no success. The most I managed was 20 photos, but it wouldn't add any more. Occasionally 1 or 2 random ones from my selection will add but that's it. My iCloud has plenty of space. Does anyone have any suggestions, beginning to get very frustrated? Many thanks!