Why won't the top layer of my hair curl up?

So, my hair used to be really curly all over, but nowadays, only the bottom layer of my hair will kink up! It's really aggravating to have half my hair straight and the other half curly. I used to have my hair layered but the layers grew out....is that what causes it? If not, is there anything else i can do to get my curly hair back? :( please and thank you!