Won't show all integers in string, please help

[fenhopi]

I have a code where I search a text file for for example: "2320018200000", then it looks it up and finds a line like this:
"3 1604001Sf nyretalg 20 kgs frys00031000100142Test før: 00365001800009000000000000000000000000010000000Sf nyretalg 20 kgs frys0000000 2320018200000"

I want it to put that entire line in a label, but for some reason it skips the "3 1604001Sf" in the start. Here's my code:

Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
        Dim test = FindStringInFile("C:\Users\danl.FATLAND\Documents\vare.txt", TextBox1.Text)
        If test Then
            Dim cnstart = TextBox1.Text
            Dim lines = IO.File.ReadAllLines("C:\Users\danl.FATLAND\Documents\vare.txt")
            Dim ComputerName = (From firstline In lines Where firstline.Contains(cnstart)).First.Substring(cnstart.Length)
            Label2.Text = ComputerName

        Else
            MessageBox.Show("not found")
        End If


    End Sub


    Public Function FindStringInFile(ByVal Filename As String, ByVal SearchString As String) As Boolean
        Dim Reader As System.IO.StreamReader
        Reader = New IO.StreamReader(Filename)
        Dim readalllines As String
        Try
            While Reader.Peek <> -1
                readalllines = Reader.ReadToEnd()
                If InStr(readalllines, SearchString) > 0 Then Return True
            End While
            Reader.Close()
            Return False
        Catch ex As Exception
            MessageBox.Show("Exception: " & ex.Message)
            Return False
        End Try
    End Function

Thanks in advance for your time.

[jmcilhinney]

It's doing exactly what you're telling it to do.

.Substring(cnstart.Length)

If you want the whole string then why are you telling it to get a substring?