S'synth VST's Not Playing

I can't figure out this mystery.  I'm using SampleTank2.5, and another synth.  I'm getting sound from the MIDI events going to my external keyboard (Alesis Fusion 8HD), but no sound going to ST2.5, or other synths, even though the meters are working on BOTH the MIDI channel, and the Audio ch. going to the softsynths.  I've tried playing the synth via the virtual kybd on the screen, but that won't play either, despite the fact I can see the internal meters working on the softsynth as well.  I vfd the driver was set up properly between my interface, and ST2.