Aux reverb track

I'm trying to set up a stereo aux track with reverb, to send to my headphone mix.



I'm using PT 11 with an MBox 3gen.



All of my instrument tracks have a prefader send to bus 3-4.



The input on my headphone mix track is bus 3-4. Output is S/PDIF. Everything works fine with that.



I set up a new vocal track with a prefader send to bus 1-2.



I set up a stereo aux track with the input set to bus 1-2; send is bus 3-4. I dropped a reverb plugin on this aux track.



I can hear the vocal track, but the aux track doesn't seem to do anything.



Any ideas? I'm new to PT 11 (recent graduate from M-powered versions).



Sorry if this is a noob question.