Checkbox auto uncheck

I've a data-grid with three radio button columns and one check-box column.

<dataGrid:DataGridTemplateColumn Header="Local"  Width="100">
<dataGrid:DataGridTemplateColumn.CellTemplate>
<DataTemplate>
<telerik:RadRadioButton Name="rdbtnLocal" GroupName="{Binding ToServiceAreaId}" IsChecked="{Binding Path=IsLocal, Mode=TwoWay}"  Width="15" Height="15"/>
</DataTemplate>
</dataGrid:DataGridTemplateColumn.CellTemplate>
</dataGrid:DataGridTemplateColumn>
<dataGrid:DataGridTemplateColumn Header="Same" Width="100" >
<dataGrid:DataGridTemplateColumn.CellTemplate>
<DataTemplate>
<telerik:RadRadioButton Name="rdbtnSame" GroupName="{Binding ToServiceAreaId}" IsChecked="{Binding Path=IsSame, Mode=TwoWay}"  Width="15" Height="15" />
</DataTemplate>
</dataGrid:DataGridTemplateColumn.CellTemplate>
</dataGrid:DataGridTemplateColumn>
<dataGrid:DataGridTemplateColumn Header="Different" Width="100">
<dataGrid:DataGridTemplateColumn.CellTemplate>
<DataTemplate>
<telerik:RadRadioButton Name="rdbtnDiff" GroupName="{Binding ToServiceAreaId}" IsChecked="{Binding Path=IsDiff, Mode=TwoWay}" Width="15" Height="15"/> 
</DataTemplate>
</dataGrid:DataGridTemplateColumn.CellTemplate>
</dataGrid:DataGridTemplateColumn>
<dataGrid:DataGridCheckBoxColumn Width="50" Header="" Binding="{Binding Path=isChanged}" IsReadOnly="False"/>

The problem is, when I check a check-box and then I change radio-button selection, check-box unchecked automatically. I don't understand why its happening. Even when If I check any other check box, already checked check-box unchecked automatically. Means I'm unable to check 2 or more check-boxes at a time.

Can anyone tell me where I'm wrong here? Any kind of help will be appreciated.