Performance or DYI/stripboard? If the former, which trimpot are you tweaking – there’s the reference voltage trimmer (RV3) CTorp pointed you to (which is used by the octave selector and the fine tuning knob), but also coarse tuning/centre note (RV6), tracking (RV1), and high frequency adjustment (RV8) and they all do different things.
Recommended order afaik is to trim RV3 with a multimeter, then use the octave selector and tuning trimmer (RV6) and knob (RV7) to dial in a mid note (datasheet suggests ~200 Hz or G3), then use the RV1 trimmer to make sure an octave is an octave (using a keyboard or the selector), and finally go as high as you’d like (datasheet suggests 10 kHz) and use RV8 to deal with any flatness. Once that’s done, you can set RV7 to a mid position and use RV6 to tune to scale.
(If you cannot get down to reasonable notes after trimming RV3, checking the voltages, and turning the octave selector and the tuning knobs to their lowest positions, you may have a bad frequency cap (C6))
EDIT: If you’ve built the stripboard layout, you only have coarse tune (same as RV6) for the first step, and the 10k trimmer (same as RV1) for the second step. C6 there is the 1nF next to the 3340.