Hi again,
I checked around a little bit, and I see this:
If you upload images directly to the media library from Media -> Add New, you will need to attach them to a post or page before they will appear in a slideshow.
I'm not sure if that's helpful for your purposes - I do know that I helped a user earlier who needed to attach his images to a post before they would function in the gallery correctly, and it seems there's something similar at play for slideshows.
So you can attach images that are currently unattached by going to your media library and clicking on "unattached" then selecting any unattached images and using the "bulk actions" drop down to attach them to a post.
That should help, but let me know if you have any lingering issues and I'll be happy to see what I can do to help.