OnSelectedIndexChanged no working in aspx Page - vb.net

Question

i have this dropdownlist in my page
<asp:DropDownList ID="ddlselectTaxas" runat="server" AutoPostBack="True" CssClass="form-control" OnSelectedIndexChanged="ddlselectTaxas_SelectedIndexChanged"
EnableViewState="True" ViewStateMode="Enabled"></asp:DropDownList>
and this is the function in the vb file
Public Sub ddlselectTaxas_SelectedIndexChanged(sender As Object, e As EventArgs)
taxas.TryGetValue(ddlselectTaxas.SelectedItem.Text, valorTxt.Text)
valorTxt.Text = valorTxt.Text.Substring(0, valorTxt.Text.Length - 2)
End Sub
I want to change the value of a textbox (valorTxt) when I select a diferent value in the dropdownlist, but the function is not firing. I dont know why, i have zero experience with VB and asp pages. I apreciate your help. thanks in advance.
EDIT
here is the full code of the page
Public Class Emitir
Inherits System.Web.UI.Page
Public taxas As New Dictionary(Of String, String)
Protected Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load
If Not IsPostBack Then
CarregaValores()
valorTxt.ReadOnly = True
End If
End Sub
Private Sub CarregaValores()
Dim dt As DataTable
Dim objAcesso As New AcessoBD
Dim consulta As String = "SELECT TE.COD_TAXA, TE.DESC_TAXA, TV.VL_TAXA FROM TAXAS_EXPEDIENTE TE JOIN TAXAS_EXPEDIENTE_VALOR TV ON TE.COD_TAXA = TV.COD_TAXA"
dt = objAcesso.DataTable(consulta, CommandType.Text)
For Each linha As DataRow In dt.Rows
taxas.Add(linha.ItemArray(1), linha.ItemArray(2))
Next
valorTxt.Text = dt.Rows.Item(0).ItemArray(2)
valorTxt.Text = valorTxt.Text.Substring(0, valorTxt.Text.Length - 2)
ddlselectTaxas.DataTextField = "DESC_TAXA"
ddlselectTaxas.DataValueField = "COD_TAXA"
ddlselectTaxas.DataSource = dt
ddlselectTaxas.DataBind()
End Sub
Protected Sub ddlselectTaxas_SelectedIndexChanged(sender As Object, e As EventArgs)
taxas.TryGetValue(ddlselectTaxas.SelectedItem.Text, valorTxt.Text)
valorTxt.Text = valorTxt.Text.Substring(0, valorTxt.Text.Length - 2)
End Sub
End Class

Answer 1

You don't show your code to load up the drop list.
Remember, your page load code fires EVERY time on post back.
So, if in code on page load, you load up the drop list? You need to do this:
Protected Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load
If Not IsPostBack Then
LoadData()
LoadCal()
End If
End Sub
Sub LoadData()
Dim cmdSQL As New SqlCommand(
"SELECT ID,(FirstName + ' ' + LastName) as EmpName
FROM Employee ORDER BY FirstName")
lstEmployee.DataSource = MyRstP(cmdSQL)
lstEmployee.DataBind()
End Sub
Function MyRstP(cmdSQL As SqlCommand) As DataTable
Dim rstData As New DataTable
Using conn As New SqlConnection(My.Settings.TEST4)
Using cmdSQL
cmdSQL.Connection = conn
conn.Open()
rstData.Load(cmdSQL.ExecuteReader)
End Using
End Using
Return rstData
End Function
Now, in above, I was using a listbox, but they work the same as a drop down list (combo box).
but, if you do NOT place the loading up of data inside of the If Not IsPostBack?
Then what happens is you select a drop down, and say click on a button (or in your case autopostback = true, then the page load FIRES FIRST and THEN your code runs!!!
and, if your code re-loads the drop list? Then your selection is lost!!!
So, for just about ANY page you create, ALWAYS, but always use the is postback code stub in on-load - and thus your loading of controls data ONLY occurs one time. Since if you re-load every time, then you lose your changes.

Answer 2

Well, I am not sure of what was happening, but I remade my page and it is working now. something was blocking the event. Thanks for your help.