[RESOLVED] Excel won't quit

[dethredic]

So I have a little gui that will read a given spreadsheet and list all the "valid" sheets. This happens when a user presses a button.

here is my code:

Private Sub bttnScanWorksheets_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles bttnScanWorksheets.Click
        If System.IO.File.Exists(TextBox1.Text) = True Then
 
            'Clear checkbox area
            CheckedListBox1.Items.Clear()
 
            ' Open excel
            Dim excelapp As Excel.Application
            Dim WB As Excel.Workbook
            Dim WS As Excel.Worksheet
            excelapp = New Excel.Application
            WB = excelapp.Workbooks.Open(TextBox1.Text)
            Dim CheckValid As Boolean
            Dim SheetsAdded As Boolean = False
 
            ' Cycle through each sheet
            For Each WS In WB.Worksheets
                Try
                    CheckValid = True
                    If WS.Range("B4").Value.ToString <> "Employee Name:" Then
                        CheckValid = False
                    End If
                    If WS.Range("H4").Value.ToString <> "Week No.:" Then
                        CheckValid = False
                    End If
                    If WS.Range("F8").Value.ToString <> "Cost" Then
                        CheckValid = False
                    End If
 
                    ' Add the sheet to the list
                    If CheckValid = True Then
                        CheckedListBox1.Items.Add(WS.Name) ' Populate the checkbox area
                        SheetsAdded = True
                    End If
                Catch ex As NullReferenceException ' Catch errors that arrise from merged cells.
                Catch ex As Exception
                    MsgBox(ex.ToString) ' any other errors will be caught here
                End Try
            Next
 
            If SheetsAdded = False Then
                MsgBox("There were no valid sheets in this workbook.")
            End If
 
            ' Close Excel
            WB.Close()
            excelapp.Quit()
            Marshal.ReleaseComObject(excelapp)
            excelapp = Nothing
            GC.Collect()
            GC.WaitForPendingFinalizers()
            GC.Collect()
        Else
            MsgBox("That file does not exist")
        End If
    End Sub

The problem is that Excel.exe doesn't quit until the program closes for some reason. This causes the spreadsheet to remain read only until the program is closed.

Any ideas why excel isn't closing properly?