I have two installations of SourceTree for Windows. One of them is working correctly; it automatically detects new versions and has the "Check for Updates" button on the Update tab of the Options page is enabled.
The other one is not detecting new versions and the "Check for Updates" button is disabled. I don't know how to fix it. It is currently stuck on version 3.1.3 and the most recent version is 3.1.5.
Unfortunately, the sourcetreeapp.com web site is two versions behind, so I don't have the option of downloading the newest version manually.
The MSI installation of Sourcetree for Windows is intended for corporate distribution and disables automatic updating so administrators can have more control and flexibility. This is likely the source of what you're seeing.
I'll ping folks about the visibility of 3.1.5 for downloading on the website, thank you for the note.
Principal Developer, Sourcetree