I just finished an animation of the Hulk. When I try to render the animation, the top of the head and the hair don't show up even though they do in my workspace. I asked another question regarding how the hair mesh on hulk doesn't move properly with my armature, and I have a feeling this has something to do with it. I temporarily fixed that issue by creating a skull and turning on "Bone Envelopes".
Here are some pictures illustrating my issue: