iReport打印显示空白页，重启Tomcat服务器后，打印正常，什么问题？

我用的iReport1.1.1版本
iReport打印显示空白页，重启Tomcat服务器后，打印又正常了。过了几天，打印又显示空白页。又需要重启服务器。
调用方法：

conn = DBConnectFactory.getInstance().getConnection("jdbc/FtnScm");
String path = "ireport/orderImport.jasper";
   File reportFile = new File(servletContext.getRealPath("/") + path);
parameters.put("orderId", vsupOrder.getId().getOrderId().toString());
parameters.put("orderNo", vsupOrder.getId().getOrderNo());
byte[] bytes = JasperRunManager.runReportToPdf(reportFile.getPath(), parameters, conn);
   response.setContentType("application/pdf");
   response.setContentLength(bytes.length);
   ServletOutputStream ouputStream = response.getOutputStream();
   ouputStream.write(bytes, 0, bytes.length);
   ouputStream.flush();
   ouputStream.close();