Adding dynamic options to combo


I am trying to put dynamic options to a predefined combo cell in the grid. The scenario is this,
When user click on the combo box it gets the unique id from prior cell to the combo and get some values.
I have to put those values in to the combo options.

My code is below

var asin = mygrid.cellById(row_id, 4).getValue();
					var dataDict = new Object();
					dataDict["asin"] = asin
						data : dataDict,
						type: "post",
						url:  "index.php/amazon/findMskuForAsin",
						timeout: 55000,
						cache: false,
						error: function(a, c) {
						  alert("The server has caused an error please try again");
						beforeSend: function(r) {
						success: function(r) { 
							mskuCombo = mygrid.getCustomCombo(row_id, 5);;
							msku = jQuery.parseJSON(r);

Now every thing goes fine but I cant put the dynamic value in first attempt. I mean when user first click the combo nothing changed in the drop down. But on the very next attempt I can find the values in the drop down. For your information dynamic values has been retrieved on very first click on combo but I don’t know why is it not displaying there and it display on the second click.

I hope I deliver the question.

Thanks in advance