This is a common problem, what's happened is there will be some corrosion on the calliper behind the stainless metal piece which has pushed it out causing the pad to be a tight fit. What I did when I fitted my new pads was to just grind the metal back of the pad down a bit so it fits, though not too much as to make them a loose fit. Either that or remove the stainless metal piece and clean the corrosion off, but there a pain to remove with that little Allen key bolt.