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 1mm thick...)
+
-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)
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 1mm 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)