Since WordPress does a major upgrade every so often, it’s important to understand that the plugins you use with wordpress may not upgrade automatically as a result. This is not, perhaps the fault of anyone in particular, it’s just that minor glitches are destined to manifest themselves occasionally.
In the case of the recent WordPress 3.0 upgrade, and likely future upgrades from WordPress, I found that certain plugins would not upgrade automagically via the WP back-end, regardless of the permissions settings on the directory – most likely the ftp session is cutting-out at some point and certain files are being skipped.
Fear not, there is a very simple fix – just go to the plugin’s homepage on WordPress.org and download the latest version, as opposed to trying an install via your admin.
Download packs are normally zipped files, so when you’ve finished downloading, unpack to your local ‘wp-content/plugins’ directory, overwriting any that are already there and upload to the same location on your server (eg: /home/your-www/public_html/wp-content/plugins), overwriting any files/folders that are already in place.
This will fix any issues you previously had with upgrading the plugin. It’s common sense really, but as with most things, the simplest solutions often get overlooked.