If you are having trouble with activation then your purpose is poor. You've not clarified a purpose.
Preview and postviewing is intended to help build curiosity and find places in the book where you can get the comprehension that meets your purpose.
I suggest you work on your activation skills on books not related to schooling or studies. Take books that interest you of about 200 pages. Learn how to set your purpose, ask mind probing questions on other books first.
I also suggest you use mind mapping when it comes to subjects that you need to remember at a later date. The mind maps make reviewing and memorising what you learned easier.
Alex
