What the dermatologist said before was that the pigment cells (melanocytes) are distributed unevenly in everone's body. For some ppl they are distributed less in certain areas, such as your face, feet, hands, and chest areas. That could be why it's harder for you to achieve a tan there.

Some ppl get one of those at home facial tanners and aim for the more difficult areas and tan them more than other places of their body.