Hey I just got a new theme and am trying to regenerate thumbnails. I'm having trouble though. I tried using the AJAX Thumbnail Rebuilder but for 5 hours it just said reading attachments. Then I tried the normal rebuilder but the vast majority of images came back as undefined. Any ideas?
Kindly re-check if there are any images showing in your media library and also the new theme, there is a possibility that the theme uses custom field to store image url.