I bought a used YAMAHA P-95. All the other keys are good except the last 3 and the 2 flats. I watched a 2 part Youtube video and took it apart ,cleaned it up. Took off the Rubber contact strip,cleaned good and the board they touch with electronic contact cleaner. It was pretty clean inside other than some pet hair and ( 1 penny and 1 dime.) Everything else plays fine.
It has to be the last 6 keys if it is an electronic issue - all keys are scanned in groups of 6 except the first 4 on the far left.
If all 6 keys are playing low volume you are having an issue with that scan line. Check the MK-H board and follow its traces pertaining to the last group of 6 - they are easily visible.
ALSO - try a Factory Reset - power off - hold down right-most key - power on whilst holding key down
Reset did not work. Looking like i will get a new board for it. GHL88 is on the board.
Is there a revised version of it? This came off a UK website.
WF21280R Upper circuit board for Yamaha GHL88L key bed.
Revised version of WF212800
YDP131, YPP625, P70, P85, P95, DGX620, ((( P125 ))), P35, P45, P105, P115, P141, P142, P135, P143, DGX640
((( P95 ))) not p125 
Completely interchangeable - I have never really been able to establish why YAMAHA does these part number changes. My best guess is that these are from a different production run and possible even made by another supplier or made from different materials.
As an aside - I’d suggest you replace all 3 of them at the same time … I usually find that one failing board, usually by corrosion of some kind, the others are possibly affected as well. The cost for all 3 is not that much higher considering the freight cost on top of the purchase price.
Keep in mind that this issue might not be the fault of the keyboard, but from aging ears. I have come across this problem (more than once) where the owner is getting up in age (usually 70+) and has lost hearing in that frequency range, usually the last octave). This may cause the illusion of softer volume, or no sound at all- (in worst case scenarios). Try a good pair of headphones or have a younger person play and listen. Then have your hearing checked to make sure…