Hey Chris,
let me clarify – only the images that are uploaded anew to a particular post are displayed in the gallery? If that is the case, then it’s the intended behaviour – you see, in WordPress only the images that are uploaded to the particular post become its attachments, otherwise (e.g. if you include them via a media gallery query) they do not become associated with the post; not sure why WordPress authors did it like this, yet it places certain limitations as to how I can render e.g. image gallery posts. Since the images for the gallery are sources from attachments (to this particular post), the gallery will only show the images that are being uploaded directly in that post; please note that it is not necessary to even insert them INTO the post – just uploading them in the post and saving changes should be sufficient. You can control which images are attached to which post in the “Media” admin menu.
In case I misunderstood your question, please let me know!
Sincerely,
Andrew / Satori Studio