Checkbox not retaining checked value

Question

I have an update panel, a modalpopup extender & a gridview. The Gridview is inside the modalpop on a panel & has a checkbox for each record whose checked status i want to retrieve using javascript. (Im actually doing a validation where a user has to select atleast 1 record from the Grid View or else an alert message will be displayed when the OK button is clicked).

Here is the sample javascript code:

function fnValidateAdd() 
{  
   var gvET = document.getElementById("ctl00_ContentPlaceHolder1_grvProof");   
   var rCount = gvET.rows.length;   
   var rowIdx = 1;

      for (rowIdx; rowIdx <= rCount - 1; rowIdx++) 
      {
       var rowElement = gvET.rows[rowIdx];  
        var chkBox = rowElement.cells[1].firstChild;   
       if (chkBox.checked)
       {  
           return true; 
       }
      } 

         alert('Select a Proof');
         return false;
      }

The problem im facing is that "chkBox.checked" always returns false for every row, even if i have checked all the records.

Has anyone encountered a similar problem?