What's your routine? I would say go one step "harsher" and see if that works. If you only CO wash, then try a non-sulfate shampoo...if you use non-sulfate shampoos, then use a sulfate shampoo...only every once in awhile, of course, as a "clarifying wash."
If you use sulfate shampoos, then get an actual clarifying shampoo (which might have stronger or more sulfates in it).

Some people will recommend baking soda, but I think even the harshest shampoo is more gentle than baking soda...but you be the judge...everyone's hair is different.

Good luck!
That's right, I said it! I wear scrunchies!!

I am a sulfate washing, cone slabbing, curly lovin' s.o.b. The CG police haven't caught me yet.