ahhh, just happens ive been here
sometimes the conductive base of the rubber skin can fail, and this can be cured with a dab of superglue and a tiny cut of kitchen ally foil :) simply glue a small square/circle of tin foil over the back base of the button skin onto the form that will press down onto the circuit board. (cutting the diddy foil is the hardest bit..)
you can test in advance with a finger tip covered with foil pressing down on the circuit board without the button skin. it should work when the foil shorts the contacts..
superglue will fail in a few months needing a re-fix.. i had to do the same with an old toshiba TV which after 30 years was still going strong but its remote had suffered from many years of use.. other glues are too thick, well were with my remote..
another option if skilled with a razor blade might be to attempt to slice off a conductive button base from a doner remote/device and then do the same with the device and swap the good for the bad again with glue..
with reference to the pics its just the round black centre of the back of the A button you need cover with tin foil to make the connection..
ps add superglue to the pad with a needle and a damp cotton bud will pick up the diddy foil cap to place into the glued pad. a magnifying glass lamp also helps me.. a drop straight from the glue bottle will be way too much and flood the area and probably dry on the surface of the foil insulating it if it dosent capture your finger too. .. (i have been there too..)
ppps
it is a fidly job that needs speed due to the sub drop volumes of superglue used, but once sussed its a 10 second fix (well just about do-able in an add break if you rush to collect all the gubbins needed..