This is by far the most common fault I encounter on the nano 6g. followed by the broken screen.
As mentioned in previous posts the issue is either the square metal shim which has shifted on the back of the button or the black dimple spot on the button itself is missing. Or Both
It's a real nuisance aligning everyting back up again with the ribber gasket but you can leave that out if you wish. It is a waterproofer mostly and only adds a little spring to the button itself.
The black dimple button is about this size "'''.'''" so it's a bit fiddly to replace. You can use something lightly bigger but no more than 1mm thick really.