You should first double check if batch mode is enabled on RS Documents Configuration tab. If you have batch mode enabled, it should create a Document Request record for each document generation request. And there is a status there for why it may have failed. They get cleared out after processed at some point. Check that first to see if any records are there not processed.
Most likely Apex class that is scheduled to run is “GenerateDocumentsBatch” though it might be one of the other Apex classes are apart of RS Documents package so you can try to check all Apex jobs.