Ketosis is a alternative form of energy for the body that runs off of fat. All glucose stored in the body, which is supplied by carbs, must be used up before the process of ketosis can begin.
The more one is keto adapted, the less time it takes to re-enter ketosis. Carbohydrate intake has to be severely restricted to less than 50 grams per day. One cannot eat large amounts of carbohydrate and protein (yes, too much protein can kick you out of keto is), and expect to enter ketosis regardless of what the ketone meter reads.