Example works, but doesn't work with the 3D object that I created (Android)

I'm new to Unity, just starting to test on the examples.

The example works fine on my phone. But I added a cube and a whale onto the image target, ran the app again, I could still see the teapot but neither the cube nor the whale.

And also when I replace the example ImageTarget "stone" with my own picture, it doesn't work either.

Could anyone help me figure out what's wrong?

Thanks a lot in advance.