When a control is nested in a RadPanelItem, it is wrapped in another RadPanelBarItem. The DataContext of the second level item is not inherited correctly. As a workaround, you can bind directly to the element which holds the DataContext with an ElementName binding. 

Setting a RadGridView element instance as content for a RadPanelBarItem, causes the RadGridView to not show all of its columns.

As a workaround to this, you could set the IsScrollViewerInsideItemsEnabled property of the RadPanelBar element, to True, and you could control the expanded length of the PanelBarItem which holds the RadGridView instance. This could be done by utilizing the ExpandedLength property.

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>