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.