Korg Studio Won't Drive Roland VP7

[mts777]

I bought a Roland VP7
I have 2 KORG Triton Studios, neither will drive the VP7, the midi led on the VP7 shows no activity. (I'm not new to MIDI) The channel settings are correct.
A Yamaha DX WILL drive the VP7.
The DX will also play my EMU piano module. (CH 1)
Both korgs WILL also play the same EMU module. (Shows channel settings are correct to ch. 1)
If the DX will play both, why will the Studio play one but not the other? What is different about the midi message from the Studios that won't drive the VP7? I've tried 'omni' settings, switched wires, and everything else I can imagine. The whole thing doesn't make sense. Any ideas?

Tom D