
Hi. If you're sure that it's down to the pusher not making contact then the first thing I'd check is to see if the contact on the module is bent back away from the pusher. If this is the case it's just a matter of carefully bending it to the same angle as the other contacts.
If that doesn't work try this...
The buttons sit in a tube which is pushed tightly into a hole in the case. It could be just that the button in question needs pushing in towards the module a little (not too hard but use reasonable pressure on a wooden chopping board -so as not to damage the pusher). Careful not to push too much as you need a special tool to get it out again if you do.
If you don't like the sound of that then a tiny blob of solder on either the pusher or the contact should do the trick.
All of these are with the module out of the case by the way.