Ultrasound

Does anyone know why certain ultrasounds look more clear than others?

With my first, the 12 week ultrasound was amazing and we got a great pic that you can see her nose, lips and mouth. With this one, the 12 week ultrasound you can just see the outline of the head and belly. Doc says everything looks normal, but I can’t help but worry that it’s so different?

Also, this was done with the same ultrasound tech and same ultrasound machine.