Placing the RadGridView control inside a PanelBarItem element does not display all of its rows when the GroupRenderMode is Flat.

A workaround for this is to place the RadGridView element inside a Grid layout and create a new RowDefinition with Height="Auto". 

<telerik:RadPanelBarItem>
    <Grid>
        <Grid.RowDefinitions>
            <RowDefinition Height="Auto"/>
        </Grid.RowDefinitions>
        <telerik:RadGridView/>
    </Grid>
</telerik:RadPanelBarItem>