Image tracking and ground

Hi,

I'm using vuforia and image tracking to detect an image to display some holograms around the physical object the image is attached to.
However, the detection seems to be quite limited and the child objects under the image being tracked get placed at "random" distanced from the camera. Sometimes it's several meters from the camera - even underground sometimes - and sometimes a lot closer.

Question 1:
Does anyone have any idea why this might be so? I'm developing an app for Hololens.

Question 2:
If that is a limitation of Vuforia, are there any quick and easy ways to detect the floor, so I can always place the objects at the same height above the floor?