A very kind soul on WordPress.org by the name of Robin W. replied to this question so I thought I would post the answer here in case it could help save someone the huge amount of time I spent trying to work out this issue. As it turns out the answer was very simple:
1. Completely and permanently delete the image that is not showing on the post.
2. Re-Upload the image with a new image name. Make sure it is uploaded as a single image and not part of a group of images.
This worked for me. Hope this helps someone else.