ListView's ItemSource (defined in XAML) not populating on initial load, works fine on refreshes

I'm on the stable channel using Xamarin.Forms 1.2.3.6257 focusing on Android. I'm relatively new to XAML, Xamarin, and the async/await pattern.

The code below is querying a web service to populate a ListView. The web service sometimes takes 5 seconds to return data so I want to display a HUD (AndHUD) message while the query is running. The HUD stuff seems to be working ok. When the app starts up the only <ContentPage Title="{Binding Title}"> appears to be bound. The ListView is empty. That tells me the web service returned data but for some reason only part of the XAML gets bound. When I click the Refresh tool bar item the ListView gets populated correctly with the HUD working as expected.

I'm populating things in OnAppearing to avoid dealing with async/await issues in the constructor.

I have less than two dozen items in each of the tab pages so things like scrolling performance aren't an issue for me now. I tried setting BindingContext in a few places in addition to ItemsSource but that didn't seem to make any difference.

What am I doing wrong to cause the ListView to not populate during the initial load?

<?xml version="1.0" encoding="UTF-8"?>
<TabbedPage 
    xmlns="http://xamarin.com/schemas/2014/forms" 
    xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml" 
    x:Class="Company.XamarinForms.Welcome" 
    Title="Welcome"
    Appearing="OnAppearing">
    <!-- http://codeworks.it/blog/?p=232 -->
    <TabbedPage.ToolbarItems>
        <!-- http://forums.xamarin.com/discussion/19225/icon-image-on-toolbar-item-in-xamarin-forms -->
        <ToolbarItem Name="Search" Activated="OnToolBarItemActivated" Order="Primary" Priority="0" Icon="ic_action_search.png" />
        <ToolbarItem Name="Refresh" Activated="OnToolBarItemActivated" Order="Primary" Priority="1" Icon="ic_action_refresh.png" />
        <ToolbarItem Name="Logout" Activated="OnToolBarItemActivated" Order="Secondary" Priority="2" />
        <ToolbarItem Name="About" Activated="OnToolBarItemActivated" Order="Secondary" Priority="3" />
    </TabbedPage.ToolbarItems>
    <TabbedPage.ItemTemplate>
        <DataTemplate>
            <ContentPage Title="{Binding Title}">
                <!--- http://blog.falafel.com/learning-xamarin-forms-and-xaml/ -->
                <ListView ItemsSource="{Binding Claims}" ItemTapped="OnListViewItemTapped">
                    <ListView.ItemTemplate>
                        <!-- http://forums.xamarin.com/discussion/25581/how-to-use-a-datatemplate-in-a-listview-in-xaml -->
                        <DataTemplate>
                            <ViewCell>
                                <!-- http://motzcod.es/post/94643411707/enhancing-xamarin-forms-listview-with-grouping-headers -->
                                <StackLayout VerticalOptions="FillAndExpand" Padding="10">
                                    <Label Text="{Binding ClaimNumber}" Font="Large" VerticalOptions="Center" HorizontalOptions="FillAndExpand" />
                                </StackLayout>
                            </ViewCell>
                        </DataTemplate>
                    </ListView.ItemTemplate>
                </ListView>
            </ContentPage>
        </DataTemplate> 
    </TabbedPage.ItemTemplate>
</TabbedPage>

`
// http://forums.xamarin.com/discussion/17388/tabbedpage
public partial class Welcome : TabbedPage
{
public bool _firstAppearance = true;

public Welcome ()
{
    InitializeComponent ();
}

// https://github.com/xamarin/xamarin-forms-samples/blob/master/Todo/SharedProject/Todo/Views/TodoListPage.cs#L101
public async void OnAppearing(object sender, EventArgs e)
{
    // HACK: prevent a refresh from occurring when the child page is popped away
    if (_firstAppearance)
    {
        await SetItemsSourceWithHud();
        _firstAppearance = false;
    }
}

public async void OnListViewItemTapped(object sender, ItemTappedEventArgs e)
{
    ClaimData.Claim claim = (ClaimData.Claim)e.Item;
    await Navigation.PushAsync(new Detail(claim));
    ((ListView)sender).SelectedItem = null;
}

public async void OnToolBarItemActivated(object sender, EventArgs e)
{
    ToolbarItem item = (ToolbarItem) sender;
    if (item.Name == "Refresh") 
    {
        await SetItemsSourceWithHud();
    } 
    else 
    {
        await DisplayAlert("Coming soon...", item.Name,"OK");
    }
}

private async Task SetItemsSourceWithHud()
{
    var hud = DependencyService.Get<IHud> (); // AndHUD
    hud.Show ("Refreshing...");
    var result = await App.GetClaimData ();
    ItemsSource = result.Summaries;
    hud.Dismiss ();
}

}
`

`
public class ClaimData
{
public List Summaries { get; set; }

public class ClaimSummary
{
    public string Title { get; set; }
    public List<Claim> Claims { get; set; }

    public ClaimSummary (string title)
    {
        this.Title = title;
    }           
}

public class Claim
{
    public string ClaimNumber { get; set; }
}

}
`

The forum doesn't seem to like my code formatting either...weird.