When I first got my hair cut as I started going curly, I had this problem. For me, it is a combination of 1) my hair being finer underneath so there is less weight to pull it down; 2) my hair hitting my collar which causes the underneath hair to bunch up; and 3) a big nasty cowlick near the nape of my neck. My fix was to take two duck bill clips stretch my underneath hair down and put one on the right and left side to hold it in place. Then I would diffuse the area for about 5 minutes. That helped A LOT.

Now that my hair has grown in a couple of inches, there is enough weight to pull down on its own except for the nasty cowlick which still puffs up my hair unless I tame it.

High Density, High Porosity, Medium Texture, 3B with some kink

Lo-Poo: No Preference
Co-wash: CJ Smoothing Conditioner
RO: CJ Strengthening Conditioner - HG
LI: CK Satin Roots
Styler: CK Coil Jam
Other Likes: Castor Oil; CK Twist Whip, MD CSC
Dislikes: Wheat Protein; Aloe LIs or Stylers