为什么我的NSURLSessionDownloadTask下载不会到达我的客户端？

My iOS app uses NSURLSessionConfiguration, NSURLSession, NSURLSessionDownloadTask, etc to download an xml file that our server produces on the fly from a database. Everything works fine with the downloads, until I try to download a larger file from one particular account. Our server has to do a lot of work before it can deliver the xml for this account. In other words, there is a somewhat significant wait before the server actually spits out the requested xml. For this largest account, the iOS client never receives the data, even though the server successfully processes the data and reports sending it to the client. My iOS app uses NSURLSessionConfiguration, NSUR