postback problem

Hello,

i am trying to change textbox value that i retrieved. I have a form which contains, a combobox from ajaxcontroltoolkit, a button to submit, a textbox for inserting comments. To beginning  when i change a combobox value, automatically, the comment stored in database is filled in textbox. Now even i change this comment, i still have older comment which is retrieved from database, new comment is never stored.

The autopostback on textbox is false. I tried to make it in true, no change. I tried to implement textchanged event. Same result.

When i add a test on ispostback property same result.

Here is some code:

            try
            {
                CbNomContact.Enabled = true;
              
                    if (cbMission.SelectedValue.ToString().Contains("Numéro:"))
                    {

                        first = cbMission.Text.IndexOf("Numéro:");
                        numSA = cbMission.SelectedValue.ToString().Substring(first + 7, first + 2);


                        CbNomContact.DataSource = BindContacts(numSA);

                        CbNomContact.DataBind();

                        CbNomContact.SelectedIndexChanged += new EventHandler(CbNomContact_SelectedIndexChanged);
                       
                          txbxdesc.Text = RetrieveServiceAppointmentDescription(numSA);

                        txbxObs.Text = RetrieveServiceAppointmentObservation(numSA);
                         
             

               }
            }
            catch (Exception e) {

                e.ToString();
            
            }

The change in combobox call this method. the submit method attached on button call implicitly again this method and overwrite the value of comment. I have a autopostback on true in combobox. I tried to make it at false, no change appear.

Any idea?

Thanks