AJAX :: RadioButtonList.SelectedIndexChanged Not Raised When The ListItem Has Selected="True" Not Working

I have two radiobuttons in radiobuttonlist and based on selection, i am enabling and disabling the text box.
First time i used date radiobutton and i took date in corresponding textbox and i saved it into database and then when i came back on same page i saw the date radiobutton is checked has proper value then i clicked on another radiobutton and this time page is postback and clear my all the data which was in my other controls(I have 5-6 text boxed on my page). I dont know why?, It supposed to select my other radiobutton and enable the corresponding text box

[Code]....

if (!Page.IsPostBack)
{
if (gvCredentialTypes.SelectedIndex >= 0)
{
CredentialType credType = CredentialType.FindCredentialType(gvCredentialTypes.SelectedValue.ToString());
if (!String.IsNullOrEmpty(credType.CredentialTypeName)) //If exists, load data
{
String expDays = credType.ExpirationLength.ToString();
DateTime expDate = Convert.ToDateTime(credType.ExpirationDate);
// Assign the value to the controls
LoadExpirationDaysDateValue(expDays, expDate);
}
}
}

private void LoadExpirationDaysDateValue(String expDays, DateTime expDate)
{
// Determines whether value for Days or Date is present
Boolean IsDays = false;
// Check if days value is present
if (String.IsNullOrEmpty(expDays) || expDays.Equals("0"))
{
txtExpirationDays.Text = String.Empty;
}
else
{
txtExpirationDays.Text = expDays;
IsDays = true;
}
if (expDate.ToShortDateString().Equals ("1/1/0001"))
{
txtExpirationDate.Text = String.Empty;
}
else
{
txtExpirationDate.Text = expDate.ToShortDateString();
IsDays = false;
}
// Set the radio button value based on whether Date or Days value is present
if (IsDays)
{
// Get ref to the 1st radio item
ListItem liExpirationInDays = rbExpiration.Items.FindByValue("rbExpirationInDays");
if (liExpirationInDays != null)
{
liExpirationInDays.Selected = true;
rbExpiration.Items.FindByValue("rbExpirationDate").Selected = false;
txtExpirationDate.Text = string.Empty;
txtExpirationDays.Text = expDays;
// Enable or disable the controls based on the radio button selection
txtExpirationDays.Enabled = true;
RequiredFieldValidator1.Enabled = true;
RangeValidator1.Enabled = true;
txtExpirationDate.Enabled = false;
RegularExpressionValidator1.Enabled = false;
CalendarExtender2.Enabled = false;
//compDateValidator.Enabled = false;
RequiredFieldValidator2.Enabled = false;
}
}
else
{
// Get ref to the 2nd radio item
ListItem liExpirationDate = rbExpiration.Items.FindByValue("rbExpirationDate");
if (liExpirationDate != null)
{
liExpirationDate.Selected = true;
rbExpiration.Items.FindByValue("rbExpirationInDays").Selected = false;
txtExpirationDays.Text = string.Empty;
txtExpirationDate.Text = expDate.ToShortDateString();
// Enable or disable the controls based on the radio button selection
txtExpirationDate.Enabled = true;
RegularExpressionValidator1.Enabled = true;
RequiredFieldValidator2.Enabled = true;
CalendarExtender2.Enabled = true;
//compDateValidator.Enabled = true;
txtExpirationDays.Enabled = false;
RequiredFieldValidator1.Enabled = false;
RangeValidator1.Enabled = false;
}
}
}

private void EnableCorrespondingTextbox()
{
switch (rbExpiration.SelectedItem.Value)
{
case "rbExpirationInDays":
txtExpirationDays.Enabled = true;
RequiredFieldValidator1.Enabled = true;
RangeValidator1.Enabled = true;
txtExpirationDate.Text = String.Empty;
txtExpirationDate.Enabled = false;
RegularExpressionValidator1.Enabled = false;
CalendarExtender2.Enabled = false;
RequiredFieldValidator2.Enabled = false;
//compDateValidator.Enabled = false;
break;
case "rbExpirationDate":
txtExpirationDate.Enabled = true;
RegularExpressionValidator1.Enabled = true;
RequiredFieldValidator2.Enabled = true;
CalendarExtender2.Enabled = true;
//compDateValidator.Enabled = true;
txtExpirationDays.Text = String.Empty;
txtExpirationDays.Enabled = false;
RequiredFieldValidator1.Enabled = false;
RangeValidator1.Enabled = false;
break;
}
}

protected void OnSelectedIndexChangedMethod(object sender, EventArgs e)
{
EnableCorrespondingTextbox();
}