I've been having a similar problem.
I've been able to deduce that the demo is using my computer's webcam, and not the Hololens camera. Rendering works on the hololens, but you have to hold the "StonesAndChips" images up to the computer's webcam in order for the teapot to appear on the Hololens device.
I actually reformatted my computer, reinstalled everything, and the problem persists. I'm on Windows 10 Pro Creators Update, Unity 5.6.2f1, the latest Vuforia package from their site, and Visual Studio 2017 Community. (Actually, VS2017 won't build...for another reason: something about missing IUnzipper.) My Hololens is up-to-date.
I've poured over every option/setting in Unity, and scoured the web to find examples of people who've previously got the samples to work.
I even tried removing the webcam from my computer. In this case, the sample runs and both teapots appear (!) off to the right in a static spot, even though there are no imagetargets to detect.
Since the Vuforia sample doesn't build in Visual Studio, I can't test that option.
It's been weeks and I can't get a reply.
I'm honestly surprised that anyone...any customer or any Vuforia developer...can get this to work on Hololens. I'm not trying to be snarky...it's just that since I started from a completely clean machine, and the problem remains, I don't know what could possibly be different on my machine compared to others'.
Best of luck to you. My only option is to postpone development and check back periodically to see if there's an update, or at least a Vuforia developer responds to any of my posts with a suggestion I can try.