IOS Shadow Disappear

Firstly thank any one who answer this question.
My problem is as follow:
I make a ios project and build light for my level.Then I find shadow disappear when I change camera rotation.

in this picture, we can see shadows normally. Then I change my camera rotation(location is as before),the shadow disappear, like this:

It seems that only in certain range rotation , can we see shadows…
I am out of idea , please give me some advices.
thank you so much.

the situation happens in IOS preview mode or installing in my ipad .
In Shader5 it is normal.


same problem. Do you have a solution ?