WinForm Bound ListBox not showing all items when DisplayMember set

I can't seem to figure out why this code won't work:

        public FormMain()
        {
            this.InitializeComponent();
        }

        public FormMain(FormMainPresenter presenter) : this()
        {
            this.presenter = presenter;

            this.listBoxProcessSteps.DisplayMember = "PageViewModel.PageTitle";

            this.bindingSource = new BindingSource { DataSource = this.presenter.Pages };

            // TODO: Why is ListBox only showing 1st page, not second. If I don't set display member, it shows both!
            this.listBoxProcessSteps.DataSource = this.bindingSource;

            this.presenter.CurrentPageChanged += this.FormMainViewModel_CurrentPageChanged;
        }

This is a MVP-VM Wizard-like application, where I am attempting to display a list of all of the pages in the wizard. Pages is of type "Collection<WizardPagePresenterBase>", where WizardPagePresenterBase looks something like:

    public abstract class WizardPagePresenterBase : IDisposable
    {
        public WizardPageViewBase View { get; set; }

        public WizardPageViewModel PageViewModel { get; set; }

        #region IDisposable implementation

        public void Dispose()
        {
            this.Dispose(true);
            GC.SuppressFinalize(this);
        }

        protected abstract void Dispose(bool disposing);

        #endregion
    }

and WizardPageViewModel looks something like:

    public class WizardPageViewModel : ViewModelBase
    {
        public string PageTitle { get; set; }

        public bool IsValid { get; set; }
    }

I have added 2 pages to my FormMainPresenter, and both of them have their PageTitle set to a valid string.

If I do not set the DisplayMember property of the list, the ToString result for both pages is displayed in the ListBox as expected. However, if I set the DisplayMember property to "PageViewModel.PageTitle", then the ListBox contains only the first PageTitle. There are no other items in the ListBox. I have set a breakpoint and verified that both PageTitles have valid strings. So, I am confused as to why setting the DisplayMember breaks the functionality.

Thanks for any help you can provide,

Todd