Augmented Reality Black Screen Problem

Hi guys
can any one help me with my problem which is i am trying to make an augmented Reality project with Vuforia and trying to use the video see through technique. first of all i am using unity 5.3.4 and Vuforia 5.5.9.
the problem is when i press the play button a black screen appears to both views but when i change the technique to optical see through it just work fine.
note: i didnt get any error message and the light of the camera that i am using to test the project is on which indicate’s that the camera is working
thanx in advance

Then why not just do this? Seems to me like you solved your own problem.

Because I am doing a study on video see through not optical see through

At least with ios/ipad’s there used to be black screen bug with vuforia… not sure if already fixed.
also some black screen issues were fixed by selecting opengl 2.0, instead of automatic graphics api

thanx for ur tip but it didnt work