Have you tested your PDF generating activity standalone manually?
For a test, did you try passing a simple html stream "<p>Hello World</p>" as the stream to HTMLtoPDF to see if the PDF is generated? If this works, then you know you need to focus on the repeating section markup to narrow this down further.
Does the parameter page on the Tracer confirm that all the params expected by the HTMLToPDF is correctly provided?
If these test still don't give you clues on how to narrow down the root cause, then you should attach the actual trace file. It will also help if you can setup this sample in a demo ruleset that we could import at our end and troubleshoot faster.