Windows Service Stops Working After Locking the Computer

I wrote a windows service in VB.NET that uses a timer to run code every so often.  However, I noticed that when I lock my computer and then log back in, the service doesn't work.  It still shows up in the Processes tab, and according to the Services manager, it is still "started".  The account is set to "LocalSystem" and the StartType is set to "Automatic".  Any ideas?  Below is a snippet of my code.

Imports System.IO
Public Class BBParserService

  Private WithEvents parseTimer As New Timers.Timer

  Protected Overrides Sub OnStart(ByVal args() As String)
    parseTimer.Interval = 30000
    parseTimer.Enabled = True
    parseTimer.Start()
  End Sub

  Protected Overrides Sub OnStop()
    parseTimer.Stop()
    parseTimer.Dispose()
  End Sub

  Private Sub parseTimer_Elapsed(ByVal sender As Object, ByVal e As System.Timers.ElapsedEventArgs) Handles parseTimer.Elapsed
    Dim fs As New FileStream("\\fileserver1\data\BBParser\log.txt", FileMode.OpenOrCreate, FileAccess.Write)
    Dim sw As New StreamWriter(fs)
    sw.BaseStream.Seek(0, SeekOrigin.End
    sw.WriteLine("testing")
    sw.Flush()
    sw.Close()
    fs.Close()
  End Sub
End Class

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