you probably arent rinsing the soap out and its leaving residue behind (dont be fooled by the bottle lol). or your scalp is producing extra gunk for whatever reason. try a deep clean.
Here is two photos just because and my dark brown hair dye has now faded to light brown but I like this color better. Also I made another pendant which came out better than the others and I like it lots.
Definitely the shampoo. You need to be careful with that shampoo, it's extremely highly concentrated and generally very difficult to rinse out. If you are going to use it I would recommend diluting it down somewhat, if you don't want to dilute it, then you really need to rinse, rinse and rinse some more to make sure you get it all out.