Steps I've done:
1. Imported the image target samples
2. Tested with play mode and it works with teapot (fig. 1)
3. Tried to import 3D model and I see it in scene mode
4. In playmode I see only the teapot...
5. Tried also to create a simple Cube, but I dont' see in the play mode. (fig.2)
With Vuforia 2.6.x (not in play mode because I use the free version of unity) the above steps worked for me.
What I've miss? I've followed exactly the "Compiling a simple project" sample.
Thank you !