You might often add more liquids or find a sauce that isn't just reducing. It can be frustrating, but you need to understand this. There are two ways to thicken your garlic sauce - heat or/and thickening agent. A thickening agent like cornstarch or flour is commonly used to thicken runny sauces. Make sure to add them one spoonful at a time, mix the sauce, and check the consistency. If you add more than required, you might have to add some more liquid and repeat the process. Allowing your sauce to sit longer on the heat to reduce itself is the reduction technique. Let it simmer if your sauce is slightly runny until you get the desired consistency.