It has to be lacking in nutrients the other parts of your hair receives. I say, pamper it and apply some kind of growth aid to it.
I don't go for growth aids like megatek, because they've never worked for me in the past. But the new bandwagon I'm jumping on is licorice after reading this article and doing lots of other research: http://riarankine.hubpages.com/hub/The-Power-of-Licorice-Healthy-Hair-Benefits
Products with licorice (even homemade products) are known to stimulate the scalp and create healthy growth.
Use this as a scalp massage on your nape, or another product you like. And pamper, pamper, pamper it to achieve the length you desire.