I don’t know about the rest of you, but I’ve experienced many instances where the Auto-update feature in WordPress fails to update either a plugin or WordPress itself.
The solution to this problem is remarkably simple, and hopefully will be fixed soon by the WP core developers in an upcoming release. The problem seems to be caused by a messy previous upgrade in which files are left in a temporary “upgrade” directory with the wp-content directory.
In order to solve the problem, you simply need to delete the “upgrade” directory in it’s entirety and then try the upgrade again. I’ve yet to find an instance on several blogs across multiple machines where this didn’t solve the problem.