Overdub mic mute during record

I am trying to overdub a vocal part and I can see that the mic is routed to the channel correctly because the track VU meter responds to sound.  Further, if I record and then play back the track the vocal is there.  But no matter what I do I cannot get the vocal into the Master or Headphone mix.  The track's output is set to 'Master' but I have also tried setting int to "Headphone" but that does not work.  I have a send on each track to the Headphone bus.  Each of the other tracks shows up in the headphone mix but the vocal mic does not (except when playing back).

To make a long story short: Why can't I hear my vocal overdub mic under any circumstances except playback?