Is it still there when your hair is dry? is it dry or is it still kind of tacky feeling?
Pics would help a bit.
Are you only washing with BS, or are you rinsing it out and then doing an ACV rinse afterwards? What are your ratios of BS to water?
If it dries and is flaky, it is probably dead skin, and you may need more BS in your solution. it if it still tacky/gooey feeling, it is probably sebum, and you need to add more BS to your solution.
There is a possibility that it is left over BS if you have way too much BS in your mixture. This would also go for if you have hard water
No matter what, you should be doing the ACV rinse after your BS wash. Unless you have naturally oily skin/hair