UK PAL System Playstation 2. Why do some games work and others not?

UK PAL PS2 SCPH-30003 R with Sony controllers. All was bought together 2nd-hand. Infuriatingly some games work fine (eg Lord of the Rings:Return of King, Heroes of the Pacific, Chronicles of Narnia, Sonic Heroes) but others refuse: Star Wars Battlefront, Conflict Global Storm, Battlefield 2, SOCOM II for example, complaining that there is no dualshock pads. What is the problem???? its driving me and my 6 year old mad. First console I've had and umed that you should be able to just plug and play without faffing about with system requirements.....Please help!!!!!