Hi and welcome to the forum.
You don't say which version of Monarch you are using or if it is the Standard or Pro Version. If you are using a Pro Version, use the JetExportTable or the JetExportSummary instead of the ExportTable command. The JetExport command is limited to the Pro versions only.
the JetExportTable command applies excel formatting to your spreadsheet and also you can append to an existing sheet and you can add sheets.
Try that and let us know how you make out.