If you have hard water, that will definitely cause it. I've had the same problem. I like what Korkscrew said. Also if you have coarse hair, usually it does not do well with protein. If you're using cones, you can't just cowash. I cowashed exclusively with hard water for a few weeks, and it just didn't work. Guess how much scum is on my shower curtain? TONS! Get yourself a clarifying shampoo, then try cowashing in between a low poo a couple times a week.
2c-3a - med-coarse - normal-high porosity - high density

NP/LP: CJ Daily Fix, KMF Whenever / Giovanni TTTT
RO/LI: Sevi Pumpkin Seed, CJ Argan & Olive Oil, YTBbs
DC/PROTEIN: KC Stellar Strands / CNPF
STYLER: Ouidad Climate Control, SS Firm Hold Gel
COLOR: henna, amla & indigo
: glycerin in high/low dews, polyquats, parabens