I think those are true stereo output and use only one of the speakers for the delay/chorus effect, leaving the other as dry signal only. So if one of the speakers is bad or disconnected the chorus effect might be non-existant. Try unplugging first one then the other to see if you still get any sound at all. According to the schematic (available at: http://www.fender.com/support/amplifier_schematics.php) it looks like the right speaker is in charge of the chorus effect.
Also the stereo effects loop contains complex normalling switching in the send & return jacks which could have dirty/oxidized contacts. Try simply plugging a cord in & out a few times to work the switching contacts. Or try a TRS (stereo) patch cord from send to return.
This is assuming you have the earlier model, rather than the Ultimate Chorus DSP.