The majority of eyebrow growth serums contain a variety of ingredients claimed to improve hair loss, such as essential oils for hair growth, certain types of protein (e.g. keratin) or vitamins (e.g. biotin for hair loss). However, there is insufficient evidence that most of these substances can increase scalp hair density or thickness when applied topically and there are even fewer studies to show their effectiveness on eyebrow growth.  This means that while these serums may have some benefits in moisturising your eyebrows and reducing unnecessary breakage, there is no proof yet they can make them grow faster, denser or thicker. The best thing you can do when experiencing slow or absent eyebrow growth is to see a hair specialist who can help you determine the best treatment.