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only a black screen with nothing

Unity3D: scene A -> AR scene (run ok)->scene A again->AR scene(error)

when i came to AR scene(error) again

1. my screen is black, but i can still track cloud images, coz i got track succeed info from xcode console.

2. my UGUI "back to scene A button" disappeard too.

3. this project works fine on editor mode, but when i run it on my pad, that error appears.

4. if i use unity change to ARScene, it will be ok, but now i need to use xcode to call UnitySendMessage, that error appears.

i've no idea about that, should i need release sth before change scene? help~~