Why did this method stop firing?

Why did this method stop firing?

I have this method:

  1. /* This causes the textbox to grow dynamically (vertically) as text/rows (lines) are added */
  2. $("[id$=explainPaymentTextBox]").keyup(function (e) {
  3.     while ($(this).outerHeight() < this.scrollHeight + parseFloat($(this).css("borderTopWidth")) + parseFloat($(this).css("borderBottomWidth"))) {
  4.         $(this).height($(this).height() + 1);
  5.     };
  6. });

...which was working fine (causes the textbox to grow vertically as necessary so that all text entered is visible), but all of a sudden, with no change of code, stopped working. Just to be sure it was firing, I added an alert:

  1. /* This causes the textbox to grow dynamically (vertically) as text/rows (lines) are added */
  2. $("[id$=explainPaymentTextBox]").keyup(function (e) {
  3.     alert("explainPaymentTextBox keyup reached");
  4.     while ($(this).outerHeight() < this.scrollHeight + parseFloat($(this).css("borderTopWidth")) + parseFloat($(this).css("borderBottomWidth"))) {
  5.         $(this).height($(this).height() + 1);
  6.     };
  7. });

And I don't see the alert! Why would the function all of a sudden stop firing and, more importanlty, how can I get it to fire again?

There *is* an element with that ID; here's an excerpt from "View Source":

  1.     
    <span class="finaff-webform-field-label" style="display:inline-block;">Explain Payment: </span><textarea name="ctl00$ctl24$g_5f3fedca_19f7_4bc3_b84e_efbef0c48a33$ctl00$explainPaymentTextBox" rows="2" cols="20" id="ctl00_ctl24_g_5f3fedca_19f7_4bc3_b84e_efbef0c48a33_ctl00_explainPaymentTextBox" class="finaff-webform-field-input" style="width:660px;display:inline-block;"></textarea><br></br><span class="finaff-webform-field-label"><h2>FOAPAL / Payment Amount Information</h2></span><br></br><button id="ctl00_ctl24_g_5f3fedca_19f7_4bc3_b84e_efbef0c48a33_ctl00_btnAddFoapalRow" type="button" OnClientClick="return false;">+</button><table border="2">