I've found that if you don't install all the available updates you currently have downloaded, the 3.1.0 updater won't show up. (Alternatively, by choosing 'delete all' in the dialog that shows the list of available updates, the list then refreshed correctly & showed the 3.1.0 update as expected.)
We're actively bugging the updater team to get this fixed.
If all else fails, uninstall Premiere Pro then after re-installing, install each update individually. Hopefully it won't come to this for you.