completion handler is not called on ios7 GM

completion handler is not called on ios7 GM

Thanks in advance.
I'm using AVAssetWriter, and it is perfectly working on iOS6.
The problem is, when I called finishWritingWithCompletionHandler, the
completion handler is not called on iOS7 GM.
I called markAsFinished, and even endSessionAtSourceTime before I call
finishWritingWithCompletionHandler.
It works fine on iOS6.
And even more, on iOS7, it works at some time. An then it doesn't work again.
I need your help.
Thanks.