Image Resizer used to work on all images on a page now only works on some
I have a products page and had all full size and thumbnail images processed by the ImageResizer successfully. When I checked the page recently I saw that only the thumbnails are being cached but not the larger images which are now being loaded from their primary location (media/images/products/img.jpg) instead of the cached location (image_cache/media/images/products/stretch/etc...)
I went so far as to delete all cached folders from the server and confirmed that only the thumbnails (cropped) are being created. I also deleted the image instance for the non-working images and reapplying it to no avail.
Any ideas as to what could have gone wrong on a single page to alter an instance of the image resizer?