As esledge informed you, the questions do indeed become more difficult as you progress through the OG books. But don't become too alarmed by the increased number of questions that you answered incorrectly. The most valuable questions are the ones that you answer incorrectly. View them as an opportunity to identify and strengthen your weaknesses, as well as reinforce the concepts tested on the GMAT. The best way to do this is through an Error Log
. If you haven't done so, please incorporate this into your studies. It will definitely help you to improve your performance.