could be because it may be damaged. A good shampoo/conditioner like joico k-pac and protein treatments can help with this, as can using the correct coloring options from a salon.
you can tell if it's damaged by feeling it. when you shower does your hair feel sponge like? If you hold your hair and run your fingers from ends to roots is it excessively rough? DOes it break off easily? all these are signs of damage. And damages hair won't hold a curl.
if it's never curled and always been thin you could just be the right type of hair that never develops a medula. (this is where curl is created, in the inner most layer of hair).
Now I can help with giving you some volume if thats what you want. Try products like a root boaster (apply in lines along the roots lightly when wet and massage into the hair). Then dry with hair and head down from scalp to ends (never point nozzle from ends to roots or you risk damage)
at 12:15 PM on May. 3, 2011