The following factors can cause your royal icing to become runny: Too Much Liquid: Too much added liquid (water or flavoring) can cause icing to become too thin. Start with a small amount and gradually add more until you reach the desired consistency. Humidity: Royal icing is sensitive to humidity. If you work in a humid environment, the moisture can affect icing consistency. Use less liquid in such conditions. Older Icing: Royal icing can separate as it sits. If you prepared the icing ahead of time and stored, remix it to get your desired consistency. To fix runny icing, try adding more powdered sugar to thicken it up. Conversely, if your icing is too thick, add a bit more liquid to thin it out. Make gradual adjustments to avoid overdoing it and ending up with the opposite problem.