I recently did some work on a client's blog. The work involved making some tweaks to their custom theme, doing staged upgrades from a very early version of WP up to current, and installing on a new host.
However, along the line, the <!--more--> tag stopped working. It was working before the upgrade, I can enter it in posts fine, but it simply doesn't activate on the blog, showing full-length posts on the front page. It doesn't work on the default theme either - so it's not a problem with the custom theme.
The tweaking did not involve editing index.php, where the_content('Read more...') tag is.
I've also tried deactivating several plugins that do stuff to the post body, and might be conflicting. No luck.
Does anyone understand the mechanism of the more tag? What could stop it functioning like this?
I've googled all over the place, but nothing fits - most of the solutions involve using an Extract plugin, which is not practicable in this case.