Best answer by kmjy
While it is possible to run your system completely wireless it can sometimes provide unexpected results. This all comes down to your network conditions, like the amount of traffic and devices on the network. Because a SONOS Home Theatre setup uses a high bandwidth connection it is mostly recommended to use a BRIDGE, BOOST, or at least one SONOS player wired into your router with Ethernet. This enables them to create their own high speed and direct wireless network which prevents any issues.
Please try plugging your SUB into your router temporarily and then through the app add it to your system again. If this does not work, plug the PLAYBAR into the router instead (if possible) and try to add the SUB again. If this works run through the Wi-Fi setup again in the Advanced section of the app to make sure all of your players are up-to-date with your wireless network configuration and settings.
If all fails you can contact SONOS Support where they can look at your systems diagnosis and determine the issue with the SUB, sometimes it's a very simple issue and they can guide you through fixing it step by step.
You can contact SONOS Support here: http://www.sonos.com/contact
Let me know how it goes,