It could be one of two things.. either the microswitches have collapsed and need replacing
or more likely the contacts have oxidised and are now making a poor connection.
Get yourself a can of deoxit, open up the JP and spray a little into each microswitch
and then wiggle each switch around to work the Deoxit into the contacts
does the trick for non responsive buttons or EM Encoders
Hope this helps
MBP 17", PC 100(Nubus Protools) Motu 896, X-Station, Logic9, Reason6, Korg legacy, ACE, Alchemy, Emax II, E-Synth, Evolver MEK, Waldorf Pulse and Blofeld, AS Telemark, AS Leipzig-S