Click event in column header never fired

I'm having a bizarre problem where I'm using identical code for two tables to retrieve the column index when the column header is clicked to change the sort. I'm doing this so I can display the sort field name in a printout. In one table it works perfectly and in the other the code for the event is never executed, as I've confirmed in the debugger. I'm using code that I found in this discussion:
https://datatables.net/forums/discussion/24593/retreive-column-idx-when-clicking-on-header. Is there something I'm not seeing here that would cause this issue?

First table (works perfectly):

  $('#tblHerbs').on( 'click', 'thead th', function () {
    var i;
    i = dtHerbs.column(this).index();
    switch (i) {
      case 0: HerbSort = 'Herb Code'; break;
      case 1: HerbSort = 'Herb Name'; break;
      case 2: HerbSort = 'Gram Cost '; break;
      case 3: HerbSort = 'Inventory'; break;
      case 4: HerbSort = 'Low Inv.'; break;
      case 5: HerbSort = 'Notes'; break;
    }
  });

Second table (nothing happens on click; this code is never reached):

  $('#tblPatients').on( 'click', 'thead th', function () {
    var i;
    i = dtPatients.column(this).index();
    switch (i) {
      case 0: PatientSort = 'Name'; break;
      case 1: PatientSort = 'Formula'; break;
      case 2: PatientSort = 'Prescription Date'; break;
    }
  });