Generally, when WP returns the base file name for a file rather than a specific size (adding -600×400 for example) it means that the original file is actually smaller than the image size that needs to be returned. It’s not easy to see by this what can be causing it, however, my guess, and the first place I’d start, is so make sure that the full size images on the server for the left are actually the full sized images and that these have not all been replaced for some reason by the smallest image from the server on the right.
