“Preview Changes” button doesn’t show changes for previously published posts/pages

When go in and make some changes to an article and click the “Preview Changes” button (without having first saved anything), another window/tab opens, but the article shown is the last saved version. That is, it’s not showing any new changes I made.

Anyone else run into this, or am I not understanding the functionality correctly?

UPDATE: Some specifics. Please note the URLs throughout.

It is a Subdomain Network install using the Domain Mapping plugin and a “pretty” permalink structure.

• Network primary domain: example.com
• Secondary site: site1.example.com
• Mapped primary domain for site1.example.com: www.another-example.com
• Domain mapping settings have both “Remote Login” and “Redirect administration pages to site’s original domain” checked. (See this forum post for explanation.)

Logged in as an Editor user, I go edit a page with a slug of “tester” (http://site1.example.com/wp-admin/post.php?post=1834&action=edit)

I make some changes to the content without clicking “Update.” Instead, I click “Preview Changes.”

A new window opens with the following URL redirects:

• site1.example.com/wp-admin/post.php
• site1.example.com/tester/?preview=true&preview_id=1834&preview_nonce=c5e51962c0
• www.another-example.com/tester/?preview=true&preview_id=1834&preview_nonce=c5e51962c0

This ends in a message saying, “You do not have permission to preview drafts.” This is annoying as the remote login redirects have not taken place.

As a work around I discovered that if I first preview at least one “draft” status post, all the remote login redirects take place. If I then go back to edit the “tester” page (http://site1.example.com/wp-admin/post.php?post=1834&action=edit) and click “Preview Changes,” I again end up at http://www.another-example.com/tester/?preview=true&preview_id=1834&preview_nonce=c5e51962c0. However, it doesn’t display any of the pending changes. Instead it shows the last saved version of the page.

Any ideas? Thank you.