Gallery Not Displaying All The Images

I recently did a set of images for a local business, and loaded these to a gallery.  For some unknown reason not all the images are showing - see attached.
All processed in the same way, so I don't understand why some are showing and others aren't.

The attached was sent to me by the client, but I see the same when I access the gallery as a customer.
If I look via admin then the thumbnails are missing, but they were no errors on the upload

Looks like they just simply didn't upload. Probably too large for the server to process. Try uploading again. If it doesn't work, make them smaller in file size and dimensions.

It's not Sytist causing them not to upload.

Issues uploading photos