Windows 8 CollectionViewSource ObservableCollection Binding Not Updating

I have a CollectionViewSource that is binding to an subset of an observable collection to show the top n articles on the main page.

<CollectionViewSource
        x:Name="groupedItemsViewSource"
        Source="{Binding Groups}"
        IsSourceGrouped="true"
        ItemsPath="TopItems" />

In my code I do the following:

protected async override void LoadState(Object navigationParameter, ...)
{
    var feedDataSource = (FeedDataSource)navigationParameter;
    this.DefaultViewModel["Groups"] = feedDataSource.Feeds;
}

My FeedDataSource contains an ObservableCollection<FeedCategory> Feeds which has an ObservableCollection<FeedItem> Items and TopItems :

public ObservableCollection<FeedItem> TopItems
{
    get { return new ObservableCollection<FeedItem>(this.Items.Take(5)); }
}

Now - I have added a refresh method which adds more FeedItems to the Items collection however when I refresh, it has no effect on the Grid View on my main page which uses the groupedItemsViewSource for it's ItemsSource property.

ItemsSource="{Binding Source={StaticResource groupedItemsViewSource}}"

If I navigate away from the page, and then back again (so the page gets rebuilt) the new items are there. Why isn't my GridView showing these changes as they happen? I thought that by binding the collection and grid view they should reflect these changes? I have tried changing them to two one, one way - but it makes no difference.

Can anyone shed some light as to why this isn't working?