I'm uploading files using the UploadFilesAsync method shown below.

```
var wc = new WebClient();
Uri myUri = new Uri(uriString);
wc.UploadFileAsync(myUri, "POST", filePath);
wc.UploadProgressChanged += new UploadProgressChangedEventHandler(wc_UploadProgressChanged);
wc.UploadFileCompleted += new UploadFileCompletedEventHandler(wc_UploadFileCompleted);

```

My UploadFileCompleted event handler is called, but for some reason the UploadProgressChanged event handler is never called. 

Any ideas why?