xaml - combobox windows phone 8.1 no dropdown -

i add combobox in xaml windows phone 8.1 in stackpanel. on running app in emulator no dropdown functionality shown. if limit stackpanel height e.g. "70", 2 first items shown. if height = "auto" items shown immediately.

how can enable dropdown functionality?

header:

    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation" xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" xmlns:phone="clr-namespace:microsoft.phone.controls;assembly=microsoft.phone" xmlns:shell="clr-namespace:microsoft.phone.shell;assembly=microsoft.phone" xmlns:d="http://schemas.microsoft.com/expression/blend/2008" xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006" 

combobox ..

<stackpanel grid.row="4" width="350" horizontalalignment="left">             <textblock x:name="playerlistpanel" textwrapping="wrap" text="select player" verticalalignment="center" margin="2,0,0,0"  horizontalalignment="left"/>             <combobox name="startplayercombobox" borderthickness="1" >                 <comboboxitem tag="player1">player 1</comboboxitem>                 <comboboxitem tag="player2">player 2</comboboxitem>                 <comboboxitem tag="player3">player 3</comboboxitem>                 <comboboxitem tag="dummy1">1</comboboxitem>                 <comboboxitem tag="dummy2">2</comboboxitem>                 <comboboxitem tag="dummy3">3</comboboxitem>             </combobox>         </stackpanel> 

try listpicker control..its functions similar combobox auto adjusting height , width..

<toolkit:listpicker> <toolkit:listpickeritem content="a+" /> <toolkit:listpickeritem content="b+" /> <toolkit:listpickeritem content="o+" /> </toolkit:listpicker> 

to use include following namespace in xaml page..add windows phone toolkit(click it)

xmlns:toolkit="clr-namespace:microsoft.phone.controls; assembly=microsoft.phone.controls.toolkit" 

if further info requires... revert back...