Could be it needs to go longer actually. An undercooked flat will taste tough and dry.
Brisket flats get their juiciness from melted connective tissue. Personally, I always wrap flats like I do chuck roasts. I'm not concerned with bark on a flat so I'll smoke it until it stalls then wrap it in HD aluminum foil with a little liquid, usually only about a half cup. Beef broth works fine. 200F internal temp usually works fine, sometimes a little less, often more. The internal temp on a flat is more an indicator when to start probing for tenderness. Probe right through the foil. You an use a toothpick, ice pick, the probe of a meat thermometer, even a two pronged carving fork. When the probe easily slides in like pushing it into warm butter, it is done. Don't be surprised if it goes as high as 207-210F Internal temp.
If you don't want to wrap the meat you can use the same tenderness and internal guidelines with the probes.