An average in the mid-60s is doing fairly well actually. I know it can be demoralizing at times, but the books are designed to have people miss about one out of every three questions so that they learn from the answer explanations. When you go back and thoroughly evaluate a question after the fact, it forces you learn the material and develop good test-taking habits. The thing about some of the BR questions is that they prey on typical mistakes people make, such as mixing up acceleration with velocity, in the hopes that the user picks up subtle nuances asosciated with a multiple-choice exam.
What will really help is if you instead of scoring how you did, look at how you'd do if you ever saw that passage (or one like it) again. If you can honestly say that you'd get 80% or higher if you were to repeat the homeowrk, then you are right on pace to get double digits in the PS section (assumign it's true for general chemistry as well).
The thing I hope you get from the books are the short cuts and strategies you encounter in the explanations and the sidenotes in the chapter. Those test strategies, if you embrace them, are amazing.