Vintage Synth Repair

If single voices are affected the problem never resides with the CPU board.
Problems with the CPU board affect all voices identically.

So we have various parameters that affect Cutoff. Most notably of course
cutoff/cutoff mod (this CV contains als the LFO modulation if applicable);
then we have the external filter control. Those two affect all voices
globally.
Then we have key track and EG intensity; those affect individual voices
(since the effect they have on a single voice depends also on the pressed
key and on the point the envelope has reached).

So if key track has no result for certain voices I would see if the
keyboard CV reaches this part of the circuit. This enters on pin 3 of IC
29. Check also the buffers between IC 29 and IC 25. Also, the buffers
behind IC 23 could also affect Cutoff in general.