求助啊 wp8中LongListSelector的问题

<Grid>                 
<!--
                    <Grid.RowDefinitions>
                        <RowDefinition Height ="*" />
                        <RowDefinition Height ="Auto" />
                    </Grid.RowDefinitions>
<StackPanel Grid.Row ="0" Orientation="Horizontal">
                    </StackPanel>
-->

                    <!--包含图像占位符和文字环绕并使用带有内容滚动的浮动标题的双线列表-->
                    <phone:LongListSelector Grid.Row ="0"> </phone:LongListSelector>

                </Grid>

为什么取消掉注释后。。longlistselector就不能上面拖动了啊？？怎么解决？？。。 这个grid在PivotItem里面
[解决办法]

<Grid>                 

                    <Grid.RowDefinitions>
                        <RowDefinition Height ="Auto" />
                        <RowDefinition Height ="*" />
                    </Grid.RowDefinitions>
<StackPanel Grid.Row ="0" Orientation="Horizontal">
                    </StackPanel>


                    <!--包含图像占位符和文字环绕并使用带有内容滚动的浮动标题的双线列表-->
                    <phone:LongListSelector Grid.Row ="0"> </phone:LongListSelector>

                </Grid>