Our spigot assembly has a very thin, flat, rubber gasket at the top of the thread that inserts into the barrel. This gasket is designed to create a proper seal all the way around the opening in the barrel. Sometimes strands of plastic around the opening in the barrel can cause a leak and they should be removed, but be careful not to enlarge the opening.
Sometimes the spigot assembly has been threaded in on a bit of an angle causing the gasket to seal on one side of the opening, but not the other side. This can be best seen when the barrel is empty and turned upside down. This way, the spigot area can be closely inspected on all sides of the opening in the barrel. The fitting can be backed out with a wrench and reinstalled in a more perpendicular position.
Sometimes everything is the way it should be and it still leaks. In this case we recommend removing the fitting, discarding the rubber gasket and using Teflon tape or silicone on the fitting threads. Once the fitting is fully installed, silicone can be used to seal the gap between the fitting and the side of the barrel. This always resolves a leak.