jQuery .on("click", handler) not doing what I need it to do

Likely an obvious mistake, but I cannot get links and functions to behave as I want them to.

Our story so far:
The .ready() function for prelist_plan_levels.html is $(document).ready(review_plan_levels);

That function's  jQuery populates the page, including some links, one of which is:

<a href="/cmdb/public_html/Work/list_plan_levels.html" class="UpDowngrade" data-pp="Nov 2015" data-scope="PP" data-updown="UP">17</a>

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Also, the first line of that function is

$('.UpDowngrade a').on( "click", listPlanLevelsClick ) ;

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On clicking one of the links like the one shown above, I'm trying to get the following (incomplete) code to work:

// =================================================================================================
// This function responds to clicks in prelist_plan_levels.html

function listPlanLevelsClick(event) 
{ 
  event.stopPropagation();
  
  alert("inside function listPlanLevelsClick()") ;
  
  var UpDown='#' + $(this).data('UpDown');
  var posted_period='#' + $(this).data('PP');
  var scope='#' + $(this).data('scope');
  
  var php_params = 'job=UpDownGrade&UpDown='+UpDown+'&posting_period='+posted_period+'&scope='+scope ; 
  
  $('#debug').html(php_params) ;

    var jqXHR = $.ajax({
      url:        '/cmdb/PHP/do_DataTables_query.php' , 
      method:     "GET" ,
      cache:      false ,
      data:       php_params ,
      success:    load__list_plan_levels 
    }) ;
}

// -------------------------------------------------------------------------------------------------
// This function is the success callback for the AJAX that retrieved UpDowngradeable data
// When the page loads its .ready() function takes over.

function load__list_plan_levels()
{
  alert("inside function load__list_plan_levels()") ;
}

// =================================================================================================
// .ready() function for page:  list_plan_levels.html

function list_plan_levels()
{
  $('#UpDownGrade').DataTable() ;
  alert("inside function list_plan_levels()") ;
}

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What happens is I get the alert for the third function, function list_plan_levels(), not either of the first two functions' alerts.
I need to do that ajax lookup and some other things before I get to that third function.

In real time I'm getting smarter WRT DataTables() and I will make some structure changes, but I still need to get past this hangup.

I've done this before, but I must be having a brain fart.

Thanks all,
Ralph