Some images not showing even though path is same as working images

I have lots of images to display on my clients site so naturally I need to FTP these. I’ve uploaded them to a folder named “/wp-content/uploads/maps/”
Yes, I know WP doesn’t like FTP etc etc. However ‘some’ of the images I uploaded (all in one batch) appear, while others do not. The paths are identical, permissions for both folders are 755 and image file permissions are all 644 by default.

I have tried disabling and reactivating plugins (testing while in both states) to no avail. I have run the ‘add from server’ plugin to register the files in WP. Nothing seems to work.

I have W3TotalCache installed and tried various configurations of that. Deactivated and reactivated.

The (developer mode) browser error (safari) shows as 404 not found. Yet they do exist on the server. I am at a loss. Can anyone suggest something I haven’t thought of?