Dropdown not firing event on usercontrol - asp.net

Question

I have a dropdown list which is inside a user control on my page.
When I select an item in a gridview it fills some data including this dropdown list selecting the correspondent value to the gridview row.
Then I can change the selection, but when i try to do that the Selected Index Changed event isn't firing. At least not at first. If i select again it fires.
I'm using asp.net with VB.
I have put AutoPostback = true, I've set the Handles in the codebehind method, I've set a default empty line and set it to default selection, but I can't get it to work.
Protected Sub Page_Load(sender As Object, e As System.EventArgs) Handles Me.Load
If Page.IsPostBack() Then
fillProd()
Dim cert As String = Session("var")
isFilled = Session("isFilled")
If Not IsNothing(cert) And Not isFilled Then
FillFields(cert)
End If
End If
End Sub
Private Sub fillProd()
dt = HttpContext.Current.Session("dt")
Dim row() As DataRow = dt.Select()
Dim dtProductos As New DataTable()
dtProductos.Columns.Add(New DataColumn("valorcombo", GetType(String)))
dtProductos.Columns.Add(New DataColumn("textocombo", GetType(String)))
Dim firstrow As DataRow = dtProductos.NewRow()
firstrow (0) = 0
firstrow (1) = ""
dtProductos.Rows.Add(firstrow)
For i As Integer = 0 To row.Length - 1
Dim fila As DataRow = dtProductos.NewRow()
fila(0) = row(i)("cod")
fila(1) = row(i)("text")
dtProductos.Rows.Add(fila)
Next
ddlproductos.DataSource = dtProductos
ddlproductos.DataTextField = "textocombo"
ddlproductos.DataValueField = "valorcombo"
ddlproductos.DataBind()
ddlproductos.SelectedIndex = 0
End Sub
Private Sub FillFields(cert As String)
Dim dtSubsData As DataTable = GetData(cert)
If Not IsNothing(dtSubsData) Then
Dim row As DataRow = dtSubsData.Rows(0)
ddlproductos.ClearSelection()
ddlproductos.SelectedValue = row("cod").ToString()
Session("isFilled") = True
isFilled = Session("isFilled")
End If
End Sub
Can anyone help?