It would help a lot if you try to collect PD4ML debug info for the cases when it generates PDF successfully and when it fails. You may switch debug info on with debug=”true” attribute of (if you use PD4ML taglib) or with pd4ml.enableDebugInfo() API call. It writes the debug to STDOUT/server’s log.

Also please send us (support pd4ml.com) a sample of PDF it generates.