The ScreenTip no longer is displayed on mouse over of the owner element. This reproduces only if the element is hovered before the ScreenTips of the previously hovered element is closed. In other words this happens if the two elements that contain ScreenTips are overlapping are very close to one another and there is no space that allows closing the tooltip while moving the mouse.

To work this around, subscribe to the Opened event of the ScreenTip control and set its Visibility to Visible if the element was previously Collapsed.

<Window.Resources>
	<Style TargetType="telerik:ScreenTip">
		<EventSetter Event="Opened" Handler="ScreenTip_Opened" />
	</Style>
</Window.Resources>

private void ScreenTip_Opened(object sender, RoutedEventArgs e)
{
	var screenTip = (ScreenTip)sender;
	if (screenTip.Visibility == Visibility.Collapsed)
	{
		screenTip.Visibility = Visibility.Visible;
	}
}