There are several things that can cause the meatballs to be dry. First, if you over-mix the meat mixture before you start, it compacts the ingredients and causes them to dry out as it cooks. Another thing would be that they are overcooked. Instead, check the temperature periodically so this doesn't happen. There is also a chance that your air fryer heats a little higher than it reads. You might need to turn the temperature down some the next time you make them.