textbox change event sheet reference will not work if another workbook open

I have a research database where the code below will react to a typed reference and return instances from the database. It all works well. My problem is that it will not function if another workbook is open. Can I force it to read the "ewc codes" sheet.

Failing that I will have to look into the workbook opening in its own instance of excel as it is very handy to have open when using other workbooks.

Any help[ appreciated

I can attach workbook if required

Private Sub TextBox6_Change()
    Dim a, i As Long, w(), n As Long, strExclude As String
    Product.ListBox4.Clear
    If Product.TextBox6.Value = "" Then Exit Sub
        If Product.ComboBox2.Value = "Yes" Then
        strExclude = "20*"
    ElseIf Product.ComboBox2.Value = "No" Then
        strExclude = "19*"
    Else
        strExclude = ""
    End If

    a = Sheets("ewccodes").Cells(1).CurrentRegion.Value
    For i = 2 To UBound(a, 1)
            If (UCase$(a(i, 3)) Like "*" & UCase$(Product.TextBox6.Value) & "*" Or _
             UCase$(a(i, 4)) Like "*" & UCase$(Product.TextBox6.Value) & "*") And _
             Not a(i, 6) Like strExclude Then

            n = n + 1
            ReDim Preserve w(1 To UBound(a, 2), 1 To n)
            For ii = 1 To UBound(a, 2)
                w(ii, n) = a(i, ii)
            Next
        End If
    Next

    If n > 0 Then Product.ListBox4.Column = w

End Sub