variable resistors...

Hello All! I fixed an equalizer here in a music center (JVC PCW330). Well, the equalizer doesn’t work and That's it - none of the five sliders have any effect at all. Not even rustles. I thought for a long time... There are two microcircuits (for each channel), both of them seemed unable to fly out, The power supply to them is normal, almost the entire circuit is in the chip - there is almost no body kit... It turned out that the contact in the resistors was _completely_ lost... In all 5, and for each channel... I wiped it with a cotton swab and alcohol - and it worked. Moreover, I actively moved them before this, because... there was an idea that there was oxide - so tearing it off did not give any effect. Why did this happen? And how long will it take for them to oxidize again? Or what was it? Best regards, Evgeniy.