Reading philosophy is a skill in itself. Allow yourself much more time to read a work of philosophy than you would to read a piece of fiction of the same length. Always read with a notebook and pen to hand. If a question occurs to you as you read (e.g. "Why is the author discussing Y when this is supposed to be about X?"), write it down. See if you can answer your question based on your reading so far. If you think you can, write down your answer. Is it an answer that the author of the work could accept? If not, then you may have the beginning of a criticism