Same problem here with Lumia 620. I used Wouter's solution and button is working perfectly again. Here's a step-by-step (...for the really clueless...) -remove cover -use paperclip to push button (...if that works cover is the problem...) -after that use paperclip to push button "up" towards the touchscreen (...gently...) -the button should NOT move upwards AT ALL (...compare with working buttons...) -mine only had a half mm tolerance wich apparently is enough to cause disfunction -i used material from a disposable plastic drink bottle (...anything aprox 0.5mm thick...) -cut to size aprox 5mm x 25mm with scissors -push the narrow side in on top of button (...between button and screen...) -insert about 5mm -test button by pushing with paperclip -if it works then cut extruding bit of plastic with scissors -use paperclip or fingernails to push plastic in completely -replace cover -PRESTO -(i applied the trick to the other functioning buttons also as a precaution)