textbox wiht databinding wont refresh!

I have a textbox "txt_studno" with databinding, e.g.

txt_studno.DataBindings.Add(New Binding("Text", ds_student, "si.si_stud_no"))

I can add a new record to the database with text in the textbox, but the problem is when I retrieve a record from the database, the textbox can't display the value. For code is:

da_StudentInfo.SelectCommand.CommandText = "Select * from si where si_app_no = 12"
da_StudentInfo.Fill(ds_student, "si")

If ds_student.Tables("si").Rows.Count > 0 Then

ds_student.AcceptChanges()
dr_student = dt_student.Rows.Find(tmp_appno)
txt_studno.Refresh
End If

Anyone knows what should I do to make the textbox can display the value?

Regards,
Calvin