I had the same problem with lumia 620. I tried the Wouter's solution but the gap was too small for me. Button was working but was bit hard to press and my last button broke because of this(I had replaced button but was still hard to press). My friends lumia 620 had a smooth press while mine was tough. So I unscrewed the top part and place some a small cardboard piece from pack of cigarettes and then put all screws back. Now the button is smooth again like new. Thank you very much.