In 4.11 "VR Preview" not displaying in Oculus DK2 - just black

Hey, My setup is UE 4.11+DK2+0.8.0 on window 7. When I hit VR Preview. it just shows black screen on dk2 as well as on desktop. Please help to find out how to get work UE 4.11+DK2 setup perfectly. Anyway I have gtx 970.