Which screens are you using ?
I know that under Windows MultiMonitor setups, Dell screens are not the easiest ones to use. Don’t get me wrong, quality of the Dell screens is in general very good, just not the easiest ones to use in such a setup. You can find some tips here: http://support.amd.com/en-us/kb-articles/Pages/DellMonitorSupportingDisplayPort12Multi-StreamTransport.aspx

Also it is best to connect hub to the Surface directly (not the dock although i have seen that work as well) and not use Active Cables to go from the Hub to the screen. The MST Hub is already active and the signal might even get to “strong” when using srealtively short active cables.