Do not use blank "campaignId" strings with Google Analytics tags
Recently, I encountered this curious problem with a client's Google Analytics Source/Medium report: the top Source/Medium by sessions was "(not set) / (not set)". This was very, very unusual. What could possibly cause GA to not recognise Source/Mediums correctly? The short answer was that a blank string had been set as the "campaignId" in the GA tags. Since GA couldn't interpret the blank string, it simply saved the Source/Medium as "(not set) / (not set)". Special thanks to Jason Packer (@jhpacker in measure.slack.com ) for his help here.