Remove at least one parameter:
exporter.setParameter(JRXlsExporterParameter.IS_COLLAPSE_ROW_SPAN,Boolean.TRUE)
Note that JRXlsExportParameter is deprecated the correct way from version 5.6 is:
JRXlsExporter exporter = new JRXlsExporter();
exporter.setExporterInput(new SimpleExporterInput(jasperPrint));
exporter.setExporterOutput(new SimpleOutputStreamExporterOutput(xlsReport));
SimpleXlsReportConfiguration configuration = new SimpleXlsReportConfiguration();
configuration.setOnePagePerSheet(true);
configuration.setDetectCellType(true);
configuration.set //The other properties you like to set
exporter.setConfiguration(configuration);
exporter.exportReport();