Used to experience this problem a lot, only had my iPhone for less than 2 months. On ringing my service provider they suggested restoring. Did same but within a couple of weeks once again found the old "will not slide" syndrome had returned. Gave the problem some thought and considered In may have been the problem. I operate a chemical manufacturing business and am continually in contact with chemicals, many oily. I've made a point of keeping my phone clean and wash my hands more diligently and since have not experienced the probblem. I think the slightest sign of an oily surface and the screen stops responding. I'm not sure how the screen responds, but does not respond to anything other than human physical contact. You can't use the screen with the back of a biro or anything else, it's not pressure sensitive but it uses something else. Anyway, consider the state of your screen and your hands, that's all. Cheers.