There is likely an impedance mismatch. The KB output 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 output 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 your external oscillator.