Flute Pitch Change problem

Hello,
I am having a little difficulty on changing the pitch.
On the attached clip of a composition that I am working on,
I have 2 Flutes playing the same notes.
But on Flute #1 (top) I changed the frequency slightly
(Properties/playback/Tuning(cents))
from 0 to 40.
If I then click on this note I can hear the pitch difference.
But on playback there is no difference in the pitch.
If I instead change the pitch from 0 to 50 I can then
hear the pitch difference also on playback.
But this pitch difference is much more than I need.
I am trying to get the resonating tone that you get
when two instruments are just slightly off tune.
Thank you!