Silverlight: Sometimes, StackPanel shows in FireFox but not IE, Why?

Question

I'm using the following XAML as part of my page:

<StackPanel x:Name="RuleWarning" Orientation="Horizontal" HorizontalAlignment="Stretch" Margin="5,0,0,0">
    <Image  Source="{Binding WarningIconPath}" Margin="0,0,0,0"  Width="16" Height="16" HorizontalAlignment="Stretch" VerticalAlignment="Center"/>
    <TextBlock x:Name="tbRuleWarning" HorizontalAlignment="Stretch" VerticalAlignment="Center" Margin="5,0,0,0" Width="Auto" TextWrapping="Wrap" />
</StackPanel>

And uses the following code to control the visibility:

if (args.Result > 0)
{
    this.tbRuleWarning.Text = string.Format("Attention: Not all rules have been processed. There are {0} objects left to verify.", args.Result);
    this.RuleWarning.Visibility = Visibility.Visible;
}
else
{
    tbRuleWarning.Text = string.Empty;
    this.RuleWarning.Visibility = Visibility.Collapsed;
}

After I deployed to a staging server, it works perfect in FireFox, but in IE, the StackPanel never shows up. What's the problem?

Thanks.

Allan