Control different instances of VST with custom interface

Hi, I have created a custom interface to control the Massive VST PugIn. It works fine with the first MIDI track, but when I add a second MIDI track with the Massive VST I cannot control it. The interface keeps controling Massive on the first MIDI track. Is there a way to do this?

Cheers, Russell

In the ISO, keys, pads modul is a button on the right above bottom bar button with the letter A and the instrument nameā€¦if you switch tracks you always have to push that button to play that instrumentsā€¦ Best regards!

Thanks, but this did not do the trick. The pads and keys do play the instrument (Midi 2), but my custom interface keeps controlling the VST in Midi 1.
Cheers, Russell