Lack of Personal Effort
There are very few students who make personal effort for preparing CSS and those who make, earn its benifit. Those who rely upon ready-made notes, written material available in the form of Dogar's, Jahangir's and Ilmi's books couldnt score well because their answer lacks novelty and personal effort. Such material is produced by almost 60% of the candidates thus examiner score them so low.
Their are the subjects that need in-depth study like EDS, Current Affairs and Pak Affairs so aspirants has to make concerted efforts to grasp the concepts. In Pakistan, 'Bloom's Taxonomy' is applied in very few fields and CSS is one among them where you have to complete all 6 steps before sitting in the exam.
P.S. Google this Bloom's Taxonomy, I loved this thing.