The SViewport needs to gain mouse capture in order to control the camera. It sounds like this isn’t happening for whatever reason. However, setting focus on the SViewport should cause it to acquire mouse capture.
It’s rather difficult to debug this via answer hub. I hope to have a dramatically easier-to-use API for handling these tasks ready by end of August. If you still need help, I’m in the #unrealengine channel on freenode (http://webchat.freenode.net/). Please be aware. I am on Korean time until ~August 14.