There is likely an impedance mismatch. The KB OUT of the Mother-32 in some circumstances may require the assistance of a buffered mult in order to properly drive one or more external oscillators over a wide range of octaves.
This should fix the issue:
1) Patch the KB OUT of the Mother-32 into the input of a separate buffered mult.
2) Then patch an output of the mult into the 1 VOLT/OCTAVE INPUT of your external oscillator.