Any clue why I'm getting these bald spots?

I'm getting these bald spots on the back of my separate scalp mesh. The hair is not combed behind the scalp, it's just not there at all. Weight painting is totally red for both density and length, and I've tried disabling modifiers like frizz and noise, and every other one. Does my model just have bad genes?

UPDATE:
The shader was the culprit! The missing hair was simply transparent, but I've switched the short hair to a simple node, and it's all a lot smoother. The complex setup is still on the longer hair.