How to Format Excel spreadsheet in a Script Task

I have a SSIS script task that formats an Excel spreadsheet - this was converted from DTS/ActiveX where I had no problems. I've checked the GAC to make sure that the Interop.Excel is there and it is. I can run this step in BIDS without a problem, but when I run this as a scheduled job it fails - or more accurately, it runs but doesn't format the spreadsheet. Since I've got 100s of similar tasks to convert, I can't continue to manually execute the process every time it needs to run. What am I missing? Here is the main part of the script:

' The ScriptMain is the entry point class of the script.
Option Strict Off
Imports System
Imports System.Data
Imports System.Math
Imports Microsoft.SqlServer.Dts.Tasks
Imports System.IO
Imports System.Data.OleDb
Imports Microsoft.SqlServer.Dts.Runtime
Imports System.DateTime
<System.AddIn.AddIn("ScriptMain", Version:="1.0", Publisher:="", Description:="")> _
<System.CLSCompliantAttribute(False)> _
Partial Public Class ScriptMain
Inherits Microsoft.SqlServer.Dts.Tasks.ScriptTask.VSTARTScriptObjectModelBase
         Enum ScriptResults
    Success = Microsoft.SqlServer.Dts.Runtime.DTSExecResult.Success
    Failure = Microsoft.SqlServer.Dts.Runtime.DTSExecResult.Failure
    End Enum
    Public Sub Main()
        Dim appExcel As Object
        Dim newBook As Object
        Dim oSheet1 As Object
        Dim oSheet2 As Object
        Dim oSheet3 As Object
        Dim oSheet4 As Object
        Dim oSheet5 As Object
        Dim oSheet6 As Object
        Dim sFilename As String
        Try
            appExcel = CreateObject("Excel.Application")
            sFilename = "Career Sales Force_Weekly.xls"
            appExcel.DisplayAlerts = False
            'Open the workbook specified
            newBook = appExcel.Workbooks.Open(sFilename)
            oSheet1 = newBook.worksheets("Current Week")
            oSheet2 = newBook.worksheets("Career Sales Force YTD")
            oSheet3 = newBook.worksheets("Current Week Students")
            oSheet4 = newBook.worksheets("YTD Students")
            oSheet5 = newBook.worksheets("Current Week FSSR")
            oSheet6 = newBook.worksheets("YTD FSSR")
            'Set Dates
            SetDates(oSheet1)
            ...
            SetDates(oSheet6)

            SetBolding(oSheet1)
            ...
            SetBolding(oSheet6)

            With newBook
                .SaveAs(sFilename, FileFormat:=56)
            End With
            appExcel.quit()

            oSheet1 = Nothing
            ...
            oSheet6 = Nothing
            newBook = Nothing
            appExcel = Nothing
        Catch ex As Exception
            Dts.TaskResult = ScriptResults.Failure
            System.Windows.Forms.MessageBox.Show(ex.ToString)
            Throw
        End Try
        Dts.TaskResult = ScriptResults.Success
    End Sub

Public Sub SetBolding(ByVal oSheetX As Object)
        Dim c As Object
        c = oSheetX.UsedRange
        For Each x In c.Columns("a").Cells
            If x.Value = "TOTAL" Or x.Value = "NATIONAL" Then
                x.EntireRow.Font.Bold = True
            End If
        Next
    End Sub

    Public Sub SetDates(ByVal oSheetX As Object)
        Dim StartDate As String
        Dim EndDate As String
        StartDate = oSheetX.Range("T4").Value
        EndDate = oSheetX.Range("U4").Value
        oSheetX.Range("A1").Value = "Weekly Data for " & StartDate & " to " & EndDate
        oSheetX.Range("U1").EntireColumn.Delete()
        oSheetX.Range("T1").EntireColumn.Delete()
        oSheetX.Range("A1").EntireRow.Font.Bold = True
        oSheetX.Range("A3").EntireRow.Font.Bold = True
    End Sub
End Class