Another possibility:

My skin is acne-prone, on my face and chest. I have to be careful about the moisturizers that I use. Though I've never gotten pimples on my scalp (lol), it is very possible that my scalp didn't respond well to comedogenic ingredients. I'm pretty positive that at least some of the ingredients in my co-wash were comedogenic. I'm pretty sure this would rule out cowashing for me in general. And again I hope I didn't do any irreparable damage to the follicles.