Session variable in controller not populating intermittently.

I am making an ajax call on grid row select from MVC View.

The action methods in the controller are called and the code executes fine, but for some reason if I place a break point on the action method, then the debugger doesn't hit it & the session variable is not populating intermittently in the controller.
Below is Ajax call -
function getchildData(rowValue) {
var row = rowValue;
url: '@Url.Action("getchildData", "Eligibility")',
context: document.body,
data: { parentTransactionId: row.find('[data-name="TransactionLog.TransactionID"]').text() },
type: "GET",
cache: true,
dataType: "html",
async: false,
contentType: 'application/html; charset=utf-8',
success: function (data) {
//error : function (data) {alert('error');},
Below is controller Action method-
public ActionResult getchildData(string parentTransactionId)
ViewModel = GetEligibilityTransactionViewModel(model, true);
Session["childdata] = ViewModel;

return PartialView("_ChildTransactions", ViewModel);