Make sure you are getting thee conditioner all the way off the scalp when you co-wash.
That is what happens a lot of times when you get the itchy scalp with being a CG'er. And you have to make sure you massage the spots that itch very well when you are washing and rinse the hair downward.
The chemical called Methylchloroisothiazolinone could be the issue. It is a preservative found in conditioners (and other products) that serve an anti-fungal and anti-bacterial. It can act as an irritant to the skin in large doses and with prolonged contact with the skin. HENCE why it's found in cosmetics and toiletries that are only in contact with the skin for a short amount of time. An example would be: Conditioners. Which we all know are meant to be rinsed out and off...and usually NOT left on the skin..in this case the scalp.
I had this same problem of a terribly itchy scalp when I became a CG'er and I wasn't sure what the problem was because I never had such itchy scalp UNTIL I became a CG'er and I had no dandruff problems at all. I did some research and found out about the preservative that I named above that is found in conditioner and other cosmetics. Sure enough.. Suave Naturals does contain that ingredient. And I am a big USER of Suave Naturals. Absolutely love the stuff! I adore the Suave Naturals Conditioner and use it more than any other. I buy the stuff in DROVES.. LOL!
Once I found that out.. I simply made sure my scalp has no conditioner on it at all. Within 2 days.. I noticed my scalp no longer itched. I massage my scalp well with the very pads of my fingers and I make sure the conditioner is THROUGHLY rinsed off my scalp. If I want to do the leave a little conditioner in my hair trick that some CG'ers do... I simply make sure the conditioner is OFF my scalp and only in my hair.
Hope this helps!