WPF 多个 MediaElement 播放总有一个无反应。

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前台：
<Grid x:Name="main">
<Grid.ColumnDefinitions>
<ColumnDefinition Width="*"/>
<ColumnDefinition Width="*"/>
<ColumnDefinition Width="*"/>
</Grid.ColumnDefinitions>
<Grid.RowDefinitions>
<RowDefinition Height="*"/>
<RowDefinition Height="*"/>
</Grid.RowDefinitions>
<MediaElement x:Name="me1" Grid.Row="0" Grid.Column="0" />
<MediaElement x:Name="me2" Grid.Row="0" Grid.Column="1" />
<MediaElement x:Name="me3" Grid.Row="0" Grid.Column="2" />
<MediaElement x:Name="me4" Grid.Row="1" Grid.Column="0" />
<MediaElement x:Name="me5" Grid.Row="1" Grid.Column="1" />
<MediaElement x:Name="me6" Grid.Row="1" Grid.Column="2" />
</Grid>
后台：
private void Window_Loaded(object sender, RoutedEventArgs e)
{
var action = new Action(() =>
{
var suffix = "wmv";
var index = 1;
foreach (var child in this.main.Children)
{
if (child is MediaElement)
{
var mediaElement = child as MediaElement;
var uriString = string.Format(@"C:\Users\Public\Videos\Sample Videos\{0}.{1}", index, suffix);
var uri = new Uri(uriString);
mediaElement.Source = uri;
index++;
}
}
});
this.Dispatcher.BeginInvoke(action);
}

[Me_Code]

这个奇葩问题到底是什么原因。