Many plugins not informing of updates

Since installing WordPress 2.5 awhile ago, I’ve only had one plugin (of over a dozen) inform me of an available update (WP Super Cache). This morning, I went looking for updates manually, and found that a handful of my plugins (cForms II, NAVT, FireStats Installer, Admin Drop Down Menus) have updates, and that most of them are indeed reflected on the WordPress Extend page (for some reason, none of the FireStats plugins are there). Some of them have had multiple updates. When WordPress 2.3.x, I would occasionally have plugins that would not inform me of updates, but it was rare (I think I noticed it happening twice).

How does WP know if there’s an update to a plugin, and why does it seem to know less often with this version? Do plugin authors have to do something new and special for WP 2.5 to recognize that there are updates? Is there something in my config that might be doing this?