Shoutbox

I am trying to make a script which relies on calling a function when the selection in a list box is changed. The code I have so far used for the function is as follows:

function OnWndTestEvent_LstBoxSelChanged(PlusWnd,ControlId)
{
if(ControlId == LstSender)
{
Wnd.SetControlText("TxtMessage", Wnd.LstBox_GetItemText("LstMessage", Wnd.GetCurSel("LstSender")));
}
}

However, when the selection is changed by clicking, the function is not called. A function for the same window being destroyed does call.

Does anyone know why this may be happening?

Thank you!