命令绑定在wpf按钮中

Question

我正在尝试在WPF中绑定一个命令，但它没有命中。 XAML中还有其他控件具有绑定功能，一切正常。无法弄清楚我错过了什么..请帮忙

<Button Grid.Row="2" Grid.ColumnSpan="2" Name="btnStart" Content="Start Getting Data" Margin="78.4,6.4,0,0" Grid.Column="1" Command="{Binding StartPolling}" />
<Label Grid.Row="2" Grid.Column="4" Content="{Binding Path=LastRefreshed}"></Label>

 public ICommand StartPolling
        {
            get
            {               
                    _startPolling = new RelayCommand(
                        param => StartPollingWell());
                } return _startPolling;
            }
        }