dispatch_async无法正常工作

Question

我在ios swift中捕获图像并尝试在后台任务中保存图像和缩略图

使用此代码

 dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_BACKGROUND, 0), {

        var imageSaveCall = self.storeReceiptImageToFile(imageToSave,fileName:fileName)
        dispatch_async(dispatch_get_main_queue(), {
            println("Raw image saved")
        })

    })
 dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_BACKGROUND, 0), {

        var imageSaveCall = self.storeReceiptThumbImageToFile(imageToSave,fileName:fileName)
        dispatch_async(dispatch_get_main_queue(), {
            println("thumb image saved")
        })

    })

它可以正常工作4次，之后它甚至没有达到商店图像功能。我在做什么错误？