Possible cause: You may be on a web host which runs PHP in apache mode without suexec but doesn't provide the FTP extension.
Solution: This is a fundamentally broken PHP setup, so ideally you should fix it or find a better web host. If that isn't possible, you'll have to download the .tar.gz file from the customer downloads area and use the manual upgrade instructions in docs/readme.html.

Possible cause: You may be on a web host which runs PHP in apache mode without suexec, but the FTP information you've provided at Admin Panel -> Settings -> System Configuration -> FTP Info is out of date.
Solution: Go to Admin Panel -> Settings -> System Configuration and enter the correct FTP info.

Possible cause: Neither cURL nor fopen URL wrappers is available so the archive cannot be automatically downloaded. The yourinstalllocation/servertest.php file should alert you if this is the case.
Solution: cURL or fopen url wrappers is required for numerous feautres in WSN, please find a web host which supports one of these two methods of accessing the internet.

Possible cause: A timeout or memory limit is triggered before the upgrade can complete.
Solution: If all pages of your website run slow (over a second to generate each page, as shown by the show execution time debug mode at Admin -> Settings -> System Configuration -> Debug mode), the web host may just be too slow to complete an automated upgrade. Or, there may be something in your WSN configuration causing the slowness -- ask WSN support if unsure.