Photos App Unable to upload 20% of my photos to iCloud Photo Library

Hi. I tried to upload my 42gb photo library from my iMac (2011, OSX10.11) to share across 3 iOS devices (iOS 9, photo library enabled). I have 200gb of iCloud storage. After a long upload (my broadband is slow but it did take 9 days) the Photos App tells me that I have '43 Items on This Mac Only' and that it was 'Unable to Upload 4,492 Items'. The remaining 34gb and over 15,000 work fine across my devices and new files upload and are shared. My library is mainly .JPEG but I have some files listed as .jpg and .bmp, no videos. Would anyone know how I can get the remaining files uploaded, or at best how can I view the files that haven't uploaded?