A good barbecue sauce needs to go on thick in order to soak into the meat and caramelize well on the grill. Reducing a sauce that seems too thin is one method of thickening that you should try before adding extra ingredients. Cook the sauce over a low heat for 20 to 30 minutes, uncovered, and see if that helps. If it's still too thin, you can use another method for thickening barbecue sauce.
Put one tablespoon of cornstarch into a mixing bowl.
Add two tablespoons of water to the bowl and whisk with a small whisk or a fork until the cornstarch is completely blended, creating a semi-thick slurry about the consistency of warm honey. This is an important step. If the cornstarch is not completely mixed, it will clump in your sauce, leaving small cornstarch dumplings behind. If you think the slurry is too thick, add a little more water.
Slowly pour the cornstarch slurry into the barbecue sauce and whisk in well.
Turn the heat up under the sauce pot until the sauce simmers lightly and continue stirring. The sauce needs to cook in order for the thickening agent to take effect. Cooking will also help to remove a cornstarch flavor from your sauce.
Cook the sauce, uncovered, for at least five minutes. This method requires a bit of patience. Do not keep dumping in cornstarch or you will end up with an incredibly thick, inedible mess.
Repeat with half of the amount of cornstarch if the sauce is still too thin, but do so very sparingly and only after you've cooked the first effort for the suggested time.