I have a download progress bar in a WPF application but it initially won't update. It usually works second time around once MyFile.zip already exists. Any reason for unexpected progress bar behaviour?

Code:
private async Task DownloadFile()
        {
            _dlFilePath = _solution.TempDownloadDirectory + "MyFile.zip";

            using (var client = new WebClient())
            {
                client.Headers.Add("Accept: text/html, application/xhtml+xml, */*");
                client.Headers.Add("User-Agent: Mozilla/5.0 (compatible; MSIE 10.6; Windows NT 6.1; WOW64; Trident/5.0; InfoPath.2; SLCC1; .NET CLR 3.0.4506.2152; .NET CLR 3.5.30729; .NET CLR 2.0.50727) 3gpp-gba UNTRUSTED/1.0");
                client.DownloadProgressChanged += new DownloadProgressChangedEventHandler(wc_DownloadProgressChanged);
                client.DownloadFileCompleted += new AsyncCompletedEventHandler(wc_DownloadComplete);
                await client.DownloadFileTaskAsync(new Uri(_solution.SolutionDownloadUrl), _dlFilePath);

                while (client.IsBusy)
                {
                    System.Threading.Thread.Sleep(1000);
                }
            }
        }

        private void wc_DownloadProgressChanged(object sender, DownloadProgressChangedEventArgs e)
        {
            this.Dispatcher.Invoke(() =>
            {
                pbDownloadStatus.Value = e.ProgressPercentage;
            });
        }

        private void wc_DownloadComplete(object sender, AsyncCompletedEventArgs e)
        {
            if (e.Error != null)
            {
                OutputErrorCloseWindow(e.Error);
            }
        }