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ImageTarget don't load the picture

Hello, i am just start using VuForia, i did exactly what tutorial showed. followed two youtube video. but for some reason my Image Target doesn't  load the picture i created from Target Manager. it is just white.

i am using unity 2017.1.0f3 personal.

and newest VuForia i just downloaded today : vuforia-unity-6-2-10.unitypackage (46.20 MB)

the Target is 5 start single picture. i did everything step  by step , can't find out what's wrong.  please help. thank you so much.