Pages removing Images from Gallery when exporting

I have an existing Pages document which has am images gallery in it, and is saved.

When I try to edit that document and change the images in the gallery, and then export it as a new Pages document, I get the message "Image Galleries were removed", and the resulting document when opened does not have any of the images.

Any suggestions as to why this is happening, and what I need to do the keep the images in the exported Pages document?

Thanks in advance.