Delegate method urlSession(_:dataTask:didReceive:) not being called

I'm working on building my first App with Swift 3, and I'm having some difficulty hooking into the URLSession Delegate methods. I've been reading over the docs and I believe that I have the correct protocols specified and the appropriate delegate methods defined, but for some reason, the didReceive:data method is not being called at all. What's curious is the delegate method for didReceive:response is firing and giving me the expected content length.

These are the protocols I'm specifying when I define my class:

class DataManager: NSObject, URLSessionDelegate, URLSessionDataDelegate, URLSessionTaskDelegate

Here is the creation and calling of my URLSession.dataTask:

let session: URLSession = URLSession(configuration: URLSessionConfiguration.default, delegate: self, delegateQueue: OperationQueue.main)
let request: URLRequest = URLRequest(url: requestUrl)
let dataTask: URLSessionDataTask = session.dataTask(with: request)
dataTask.resume()

And here are the delegate methods I implented:

func urlSession(_ session: URLSession, dataTask: URLSessionDataTask, didReceive response: URLResponse, completionHandler: @escaping (URLSession.ResponseDisposition) -> Void) {
    print("urlSession -> didReceiveResponse")
    self.stream.length = 0
    self.streamLength = Int(response.expectedContentLength)
    print("expectedLength: \(self.streamLength)")
}

func urlSession(_ session: URLSession, dataTask: URLSessionDataTask, didReceive data: Data) {
    print("urlSession -> didReceiveData")
    self.stream.append(data)
    self.percentDownloaded = Float(self.stream.length) / Float(self.streamLength)
    print("percentage: \(self.percentDownloaded)")
}

func urlSession(_ session: URLSession, task: URLSessionTask, didCompleteWithError error: Error?) {
    print("urlSession -> didCompleteWithError")
    print("\(error?.localizedDescription)")  
}

Can anyone help me work out what the issue is?

Am I missing something obvious/simple?