ispostback error or not?

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ASP.NET

Hello programmers, got a problem here(as always). as you see ive bind data to the 2 dropdown, the first 1 is the root of the second dropdown and the 2 dropdown is the root of the textboxes. so far no problem regarding that matter. now here is my problem when i choose another value on the second dropdown the data from it returns to the first value;
plss help.. thanks in advance and more power
Code:

Imports System.Math
Imports System.Web
Imports System.Data.SqlClient
Imports System.Data
Partial Class PlanvsAct
    Inherits System.Web.UI.Page

    Protected Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load
        If (IsPostBack = False) Then
            categorybind()
        End If
        coursecodebind()
        binddata()
    End Sub

    Public Sub categorybind()
        Dim strSQL As String
        Dim connection As SqlConnection = New SqlConnection("Data Source=ML0003135586;Initial Catalog=TestSQL;Integrated Security=True")
        strSQL = "Select [Category] from [tblCategory]"
        connection.Open()
        Dim command As New SqlCommand(strSQL, connection)
        ddcategory.DataSource = command.ExecuteReader
        ddcategory.DataTextField = "Category"
        ddcategory.DataValueField = "Category"
        ddcategory.DataBind()
        connection.Close()
    End Sub

    Public Sub coursecodebind()
        Dim strSQL As String
        Dim connection As SqlConnection = New SqlConnection("Data Source=ML0003135586;Initial Catalog=TestSQL;Integrated Security=True")
        strSQL = "Select CourseCode from tblTrainingPlan where ([Category] LIKE '%' + @Category + '%')"
        connection.Open()
        Dim command As New SqlCommand(strSQL, connection)
        command.Parameters.AddWithValue("@Category", ddcategory.SelectedValue)
        ddcoursecode.DataSource = command.ExecuteReader
        ddcoursecode.DataTextField = "CourseCode"
        ddcoursecode.DataValueField = "CourseCode"
        ddcoursecode.DataBind()
        connection.Close()
    End Sub
    Public Sub binddata()
        Dim strSQL As String
        Dim connection As SqlConnection = New SqlConnection("Data Source=ML0003135586;Initial Catalog=TestSQL;Integrated Security=True")
        strSQL = "Select * from tblTrainingPlan where ([CourseCode] LIKE '%' + @CourseCode + '%')"
        connection.Open()
        Dim command As New SqlCommand(strSQL, connection)
        command.Parameters.AddWithValue("@CourseCode", ddcoursecode.SelectedValue)
        Dim datareader As SqlClient.SqlDataReader = command.ExecuteReader
        Do While datareader.Read()
            txtTitle.Text = (datareader("CourseTitle").ToString)
            txtcollege.Text = (datareader("College").ToString)
            txtDuration.Text = (datareader("Duration").ToString)
        Loop
        connection.Dispose()
        connection.Close()
    End Sub



    Public Sub control()
        If ddcoursecode.SelectedValue = "" Then
            txtTitle.Text = ""
            txtcollege.Text = ""
            txtDuration.Text = ""
        End If
    End Sub
End Class