Thank you, Jeffrey, I had eventually resolved it this way. However, I didn't like using this method; in fact, the system is declaring a document that is actually valid and standard as invalid. Creating a custom schema to handle a standard HL7 honestly doesn't appeal to me much. Anyway, thank you very much for the response; indeed, that's exactly what I did, and it's working! :)

Hello Scott, thank you very much for the response. What do you mean by schema issue? I'm not grouping anything; the message used was taken from the official IHE Laboratory Technical Framework. I'm having the issue with a message I receive that is similar (virtually identical) to the IHE document message.