It sounds like product buildup.

I know some curlies here have problems with polyquats, and I think all of the devacurl products (or most) have them. Some have said that the polyquats have been hard to get rid of and they eventually needed a silicone-free sulfate shampoo to get rid of them. But I would try a sulfate-free low-poo first, and/or baking soda cleanse followed by apple cider vinegar rinse.

If that doesn't work I would go for a clean slate w/ a cone-free sulfate poo. You'll have to deep condition afterwards to compensate for the drying effects of the poo. Good luck!