Build up, how do I rid myself of this?
From what it looks like on geographical surveys of hard water in the US, you are right on the edge of having hard to moderately hard water.
What this means is that the BS does not dissolve completely. When you wash, some still remains. This will cause buildup.
The other issue you might be facing is that you could just not be using enough BS. What is the ration of it to water you're mixing? If not enough, it is exfoliating the dead kin on your scalp, but doesn't have enough left over to take it all the way off. It's a problem many people have before they find the perfect ratio for themselves.
Ad last, Are you doing an ACV rinse afterwards? You have to. Your scalp and hair are slightly acidc. BS is a base. It will raise the pH of your scalp and make is kind of slippery. The ACV is acetic acid, which brings your pH back to normal.