The last two projects I have been working on a VSt instrument track suddenly goes quiet.
Everything is fine then while editing something casues the volume to go really really quiet on the instrument track. You can see the led’s just registering signal right down.
We change the preset and voila! Everything is good. Is it the weather? What? 
Aloha g,
Just a shot-in-the-dark here but could your controller (keyboard ) somehow
be sending MIDI volume info to the plug?
Reason being:
I use a MIDI guitar as a controller and one day while working
for no reason certain tracks would play out of tune and
change volume.
It was not consistent and for the life out me
I could not figure out what was happening.
Then I looked down at my guitar which was resting in a guitar stand
only to see my cat rubbing against the whammy bar and moving knobs
by rubbing against the guitar.
This cat behavior was sending weird MIDI info (bend/volume) to which ever
MIDI channel I had selected in Cubase.
Dumb lil story I know but probs can come from the strangest places.
Hope you get this sorted.
HTH
{‘-’}