My lower legs won't tan, what should i do?

my skin used to tan really well when when i was like 10 and younger. now i am 19 and i CAN NOT get my lower legs (shin area) to tan! it is driving me nuts! they get a little bit of color but the rest of me is always darker. i don't want to use that sunless tanning lotion because the rest of me usually gets really good color. is there anything i could do to help them? i mean outside or in a tanning bed they just don't get color.i exfoliate and moisturize and it just doesn't seem to help melanin production so please help!?