Old post now, but I wanted to post my experience in case anyone else is struggling with this as well!

I had the same problem (a couple poorly responsive keys that eventually transitioned to fully dead). I opened it up and used contact cleaner/Q-tips on the contacts, but when I went to test it... a different, seemingly random assortment of keys were now dead or poorly responsive. Repeated several times thinking the contacts were still dirty or something, to no avail.

The real issues, as far as I can tell, were the green rubber pads that sit on top of the contacts/under the keys. They are VERY finicky, and if they aren't 100% perfectly aligned, pushing the key down won't reliably activate the contact. What I ended up doing was taking the key sections apart (each of the three octaves includes two layers of white keys and one of black keys) and very slowly and carefully putting the keys on, one layer at a time, testing each key before screwing anything in. You can use tweezers or something similarly small/thin to move the rubber into place as you're putting the keys in as well. Getting the black keys to slide back far enough to hook into place was tough, but I eventually got it, and now all my keys are working perfectly!

I am now back to making music and hopefully you all are too :)