Highly doubt it is a nozzle issue unless the engine appears to be running really uneven. You can view the evenness of the burn from the rear as the engine is running (get back about 20'). Look for concentric burn pattern around the circumference. If the burn is clean, this is not your issue.

I suggest you put the engine back in the box and send it back for testing. I can send a replacement from here if Barry does not have any available.