For the past year or so I've had an ongoing problem when loading a Zip file with a few hundred photos. Only part of the Zip file loads, and I then have to load the remaining photos. This sometimes takes as many as three or four re-Zips and uploads.
On my most recent upload today, two of the photos only loaded in Small and Original size, with those two appearing at Original size on the thumbnail page. I did nothing different when uploading those images.