When cooking your gravy, keep the heat on medium. If you try to speed up the cooking process by turning up your heat, the sugars may not melt resulting in a gravy that's just very thin! It's important to also keep in mind that you should cook the mixture until it is thickened and the whisk leaves a track (though they'll quickly disappear) in the gravy. The gravy will still seem fairly thin when it is finished, but it should coat the back of a spoon. As it cools it will continue to thicken even more.