I have had this problem for a week.
I work with Unity 5.3.0.f3 an.df Vuforia 6.2.6.
I have added my web camera to profiles.xml.
My scene si simple. ARCmera and one Image Target with a cube as child. I configured ARCamera and checked Load XXX DataBase and Activate.
I downloaded my database.
However, when I run my web camera doesn't track the image. And my android device either.
Any idea what I am doing wrong?