You are unable to update the SIX or the SI2015 Client to the latest version. The SIX or SI 2015 Client prompts to check for updates but when you check for updates, it says "Your software is up to date".
A common scenario is when your company installs SI 2015 Server and then some SIX Clients upgrade to SI 2015 without any issues while other(s) SIX Clients do not update. Another example is when a new SIX Client is installed and the SI 2015 Server was already installed.
This issue occurs because the SIX / SI 2015 Client(s) have not made an initial connection to the SIX / SI 2015 Server, but since the SIX / SI 2015 Server no longer exists, the SI 2015 server does not recognize the affected SIX / SI 2015 Client.
The solution is to re-install the SI 2015 Client. Please download and install the SI 2015 Client from the following link: http://downloads.d-tools.com/si/2015/SIClient.exe