Take a bit of time to look at the style and structure of the book that you were successful with and then compare that with the others. What you intuitively resonated with in that book is probably lacking in the others. However by noticing the differences, (language pattern, Layout, plain English approach, short concises sentences etc,) will all have an influence in how you understand something. Bt noticing what you liked in the first book, you can look for those features in successive books.
Then you may also realise that while you may be doing everything exactly the same the difference is... the book has a different author and therefore a different personality. Just like you cannot expect all your friends and acquaintances to react the same way to everything that you say or do. Nor do you hold the same expectations of each of them. The same really goes for books.
Remember books are a reflection of a person, someone with a personality that you may or may not like but a person with a point of view.
Alex
