Some items not showing up in ItemsControl via ItemsSource

I'm trying to get an ItemsControl, which contains a grid, to display 9 checkboxes with different properties. However, only three ever show up.

I have a checkbox model class which has four properties representing the checkboxs' content, grid.row/column, and isChecked properties. I then create nine of these in my viewmodel and add them to an ObservableCollection.

Next I bind the ItemsSource of my ItemsControl to the collection. Then, I add a checkbox control to the grid inside of the ItemsControl and bind the appropriate properties.

However, only the first three checkboxes are showing up and I have no idea why. I did verify via debugging that the collection being bound to does have the correct number of items.

Here's my CheckboxModel class:

public class CheckboxModel : INotifyPropertyChanged
{
    public event PropertyChangedEventHandler PropertyChanged;

    private bool _isChecked;
    public bool IsChecked
    {
        get { return _isChecked; }
        set { _isChecked = value; OnPropertyChanged("IsChecked"); }
    }

    private string _content;
    public string Content
    {
        get { return _content; }
        set { _content = value; OnPropertyChanged("Content"); }
    }

    private int _gridRow;
    public int GridRow
    {
        get { return _gridRow; }
        set { _gridRow = value; OnPropertyChanged("GridRow"); }
    }

    private int _gridColumn;
    public int GridColumn
    {
        get { return _gridColumn; }
        set { _gridColumn = value; OnPropertyChanged("GridColumn"); }
    }

    // Create the OnPropertyChanged method to raise the event
    protected void OnPropertyChanged(string name)
    {
        PropertyChangedEventHandler handler = PropertyChanged;
        if (handler != null)
        {
            handler(this, new PropertyChangedEventArgs(name));
        }
    }
}

In my view model, I create a collection of these:

private ObservableCollection<CheckboxModel> _checkBoxList = new ObservableCollection<CheckboxModel>();
public ObservableCollection<CheckboxModel> CheckBoxList
{
    get
    {
        return _checkBoxList;
    }
    set
    {
        if (null != value)
        {
            _checkBoxList = value;
            OnPropertyChanged("CheckBoxList");
        }
    }
}

public void CreateCheckboxList()
{
    CheckBoxList.Add(new CheckboxModel { IsChecked = true, Content = "Latitude", GridRow = 0, GridColumn = 0  });
    CheckBoxList.Add(new CheckboxModel { IsChecked = true, Content = "Longitude", GridRow = 1, GridColumn = 0 });
    CheckBoxList.Add(new CheckboxModel { IsChecked = true, Content = "Altitude", GridRow = 2, GridColumn = 0 });
    CheckBoxList.Add(new CheckboxModel { IsChecked = true, Content = "Depth", GridRow = 0, GridColumn = 1 });
    CheckBoxList.Add(new CheckboxModel { IsChecked = true, Content = "Speed", GridRow = 1, GridColumn = 1 });
    CheckBoxList.Add(new CheckboxModel { IsChecked = true, Content = "Heading", GridRow = 2, GridColumn = 1 });
    CheckBoxList.Add(new CheckboxModel { IsChecked = true, Content = "Roll", GridRow = 0, GridColumn = 2 });
    CheckBoxList.Add(new CheckboxModel { IsChecked = true, Content = "Pitch", GridRow = 1, GridColumn = 2 });
    CheckBoxList.Add(new CheckboxModel { IsChecked = true, Content = "VOS", GridRow = 2, GridColumn = 2 }); 
}

And then finally here's my xaml:

<ItemsControl ItemsSource="{Binding CheckBoxList}">
    <ItemsControl.ItemTemplate>
        <DataTemplate>
            <Grid>
                <Grid.ColumnDefinitions>
                    <ColumnDefinition Width="149.6*" />
                    <ColumnDefinition Width="149.6*" />
                    <ColumnDefinition Width="149.6*" />
                </Grid.ColumnDefinitions>
                <Grid.RowDefinitions>
                    <RowDefinition Height="28*" />
                    <RowDefinition Height="28*" />
                    <RowDefinition Height="28*" />
                </Grid.RowDefinitions>
                <CheckBox Grid.Row="{Binding GridRow}"
                          Grid.Column="{Binding GridColumn}"
                          Margin="14,6,63,6"
                          VerticalAlignment="Center"
                          Content="{Binding Content}"
                          IsChecked="{Binding IsChecked}" />
            </Grid>
        </DataTemplate>
    </ItemsControl.ItemTemplate>
</ItemsControl>