Why checkbox return false value?

I am using Checkbox control and getting it's value at CHeckChanged. I am getting correct value but in IF condition it return FALSE for checked even, why ?

 protected void chkOverAllApproval_CheckedChanged(object sender, EventArgs e)
    {
        try
        {
            CheckBox chkOverAllApproval = sender as CheckBox; //True i.e. correct value

                if (chkOverAllApproval.Checked == true) //return false and jump to else even for TRUE, why ?
                {

                    mdlCertificate.Hide();
                }}