I have this problem too. I haven't tried either of these solutions yet, but I have had them recommended to me and plan to try them out!
1. Try using a scalp brush when co-washing to get the scalp extra clean. Something like this: Amazon.com: Shampoo Scalp Massage Brush- 1 Brush: Beauty
2. If that doesn't work, I might try low-pooing once every one or two weeks with Neutrogena T/Sal shampoo. It does have Sodium C14 16 Olefin Sulfonate, which isn't ideal, but it might be necessary for my scalp.
3. Apparently tea tree extract can be really good for the scalp. I don't know how this fights buildup (or if it really does - anyone else know more?), but some people add it to conditioner or shampoo, or buy a product that already has it in it.
I'm mainly interested in the scalp brushes, because my hair is very dense and I don't think my fingertips are really doing the job, even though I concentrate on giving myself a thorough scalp massage while co-washing.