Diagram of classes from classes already created does not appear the relationship

When I create a class diagram in VS2017 from existing classes, the relationship link does not appear.

namespace CloudEye.Domain
{
   public class Car
   {
      public CarPlate CarPlate { get; private set; }
      public double Capacity { get; set; }
      public double Consumption { get; private set; }
      public double Autonomy => Capacity * Consumption;

      protected Car () { }

      public Car(CarPlate carPlate, double capacity, double consumption)
      {
         CarPlate = carPlate;
         Capacity = capacity;
         Consumption = consumption;
      }
  }
}

namespace CloudEye.Domain
{
   public class CarPlate
   {
      public string Characters { get; private set; }

      protected CarPlate() { }

      public CarPlate(string characters)
      {
         Characters = characters;
      }
   }
}