Help troubleshooting binding

I have two situations I believe are the same. One binds, the other doesn't. This is .net 4.6 with entity framework 6.

This one works, it binds ID_GDB successfully.

<ContentControl x:Name="CurrentItemPanel"
                Grid.Row="2"
                Content="{Binding SelectedItem, ElementName=MainList}"
                Visibility="{Binding SelectedItems.Count, ElementName=MainList,Converter={StaticResource intToVisibilityConverter}}"><ContentControl.Resources><DataTemplate DataType="{x:Type local:Game}"><DockPanel><StackPanel x:Name="GDBID_Stackpanel" Orientation="Horizontal" DockPanel.Dock="Top" Margin="10,0,0,0"><TextBlock Text="GDBID: "
                           Style="{StaticResource Heading1}"
                           DockPanel.Dock="Top"/><TextBlock Text="{Binding ID_GDB}"
                           Style="{StaticResource Heading1}"
                           DockPanel.Dock="Top"/></StackPanel></DockPanel></DataTemplate></ContentControl.Resources></ContentControl>

public partial class Game : INotifyPropertyChanged
{
    public long? ID_GDB
    {
        get { return Releases[0].ID_GDB; }
    }
}

This one does not work. It does not bind Matched.

<StackPanel x:Name="HeaderBar" Orientation="Horizontal" DataContext="{Binding SelectedItem, ElementName=PlatformList}"><TextBlock x:Name="Head2" Text="Matched: " Style="{StaticResource Heading1}" VerticalAlignment="Center" Margin="20,0,0,0"/><TextBlock x:Name="Head3" Text="{Binding Matched}" Style="{StaticResource Heading1}" VerticalAlignment="Center"/></StackPanel>

public partial class DMPlatform : INotifyPropertyChanged
{
    int Matched
    {
        get
        {
            return DMReleases.Where(x => x.ID_GDB != null).Count();
        }
    }
}

• Much code that I believe to be irrelevant is omitted.
• These are in different windows.
• PlatformsList is a ListBox bound to a List<DMPlatforms>.
• The DataContext of the HeaderBar Stackpanel is binding correctly to the SelectedItem in PlatformsList
• The Matched property returns the correct number
• I have tried many things, including rewriting the second control to be essentially word-for-word identical to the first.