Storyboard Intermittently can't find TargetName - xaml

Question

This is probably the strangest issue I have ever experienced, and I can't find any logical explanation for it.
In my WP8 app, I'm trying to animate an item off the screen. It doesn't really matter to me whether the item's width is decreased or it is moved to one side off the screen.
I've tried shrinking the item's width, placing the item in a container and shrinking the container's width, and translating the item off the screen. But, every once in a while, the storyboard throws an exception saying that it can't resolve the TargetName. This happened for every single method I tried, even though the name of the element I animated was different every time. What's even worse, I insert a breakpoint on the line that the storyboard begins and I type the name of the element into the watch and it is there. It isn't null or anything. So I have absolutely no clue what's going wrong.
Here's code from one of my storyboards (one to hide the element and one to show the element):
<Storyboard x:Name="OpenSearch">
<DoubleAnimationUsingKeyFrames Storyboard.TargetProperty="(FrameworkElement.Width)" Storyboard.TargetName="txtNearbyHeader">
<EasingDoubleKeyFrame KeyTime="0" Value="298"/>
<EasingDoubleKeyFrame KeyTime="0:0:0.3" Value="0">
<EasingDoubleKeyFrame.EasingFunction>
<QuarticEase EasingMode="EaseOut"/>
</EasingDoubleKeyFrame.EasingFunction>
</EasingDoubleKeyFrame>
</DoubleAnimationUsingKeyFrames>
<DoubleAnimationUsingKeyFrames Storyboard.TargetProperty="(FrameworkElement.Width)" Storyboard.TargetName="txtSearch">
<EasingDoubleKeyFrame KeyTime="0" Value="0"/>
<EasingDoubleKeyFrame KeyTime="0:0:0.3" Value="480">
<EasingDoubleKeyFrame.EasingFunction>
<QuarticEase EasingMode="EaseOut"/>
</EasingDoubleKeyFrame.EasingFunction>
</EasingDoubleKeyFrame>
</DoubleAnimationUsingKeyFrames>
</Storyboard>
and the element I'm trying to animate:
<StackPanel Grid.Row="0" Orientation="Horizontal" Background="#B3006242">
<TextBlock x:Name="txtNearbyHeader" Text="NEARBY" FontWeight="Light" FontSize="{StaticResource PhoneFontSizeExtraLarge}" Foreground="White" Width="298" Padding="12,5" />
<telerikPrimitives:RadTextBox x:Name="txtSearch" Width="0" Padding="6,1" Margin="0,-8,0,0" BorderBrush="Transparent" BorderThickness="2" Watermark="search for stores near..." ActionButtonVisibility="Visible" ExecuteActionOnReturn="True" ActionButtonTap="txtSearch_ActionButtonTap" KeyUp="txtSearch_KeyUp" />
<Button Style="{StaticResource LargeButtonStyle}" Padding="0" Click="btnSearch_Click" >
<Image Width="60" Source="/Toolkit.Content/ApplicationBar.Search.png" />
</Button>
<Rectangle Width="1" Fill="White" Margin="0,18" />
<Button Style="{StaticResource LargeButtonStyle}" Padding="0" Click="btnStoreList_Click" >
<Image Width="60" Source="/Assets/MainPage/List.png" />
</Button>
<Rectangle Width="1" Fill="White" Margin="0,18" />
<Button Style="{StaticResource LargeButtonStyle}" Padding="0" Click="btnFilter_Click" >
<Image Width="60" Source="/Assets/MainPage/Filter.png" />
</Button>
</StackPanel>
I'm trying to hide txtNearbyHeader and show txtSearch. The rest of the elements get pushed off the screen to the right.
If anyone has any ideas as to what may be causing this, I'm all ears!
Thanks for your help,
Elliott