Does not regen existing thumbnails.

I had a plugin installed to sharpen images after resizing but found that it was changing the color of my images. Removed/deleted the plugin, then Regenerated Thumbnails. Once complete (either globally or directly to the specific image) my site was still showing the old image. Cleared cache, etc, still there. If however I go into the wp_uploads directory and delete the thumbnails and Regenerate Thumbnails again it will work.

Do you know why this is happening and how to get it working properly without first deleting the thumbnails?