Mouseover not working after click in WPF

Question

Hi im using a button and im wrote a style for that on MouseOver to change the color
the style is working when MouseOver is there...but after i click and then MouseOver...
then MouseOver is not working...i mean color is not changing...
can anybody help
here is the code which i used
<Style x:Key="Header" TargetType="{x:Type Button}">
<Setter Property="Background" Value="#3D9828"/>
<Style.Triggers>
<Trigger Property="IsMouseOver" Value="True">
<Setter Property="Background" Value="#70af0e"/>
</Trigger>
</Style.Triggers>
</Style>