According to the documentation the Event Gain Envelope can be adjusted in small increments by:
1) Holding down the control key while moving it with the mouse.
2) By holding down the right mouse key while adjusting the sustain portion of the envelope.
1) Makes no difference, using method 2) no amount of dragging up or down makes any change to the envelope.
I've checked this in V5, and 6 on one PC and V9 on another PC, same on all of them.
Track volume envelopes adjust as advertised.
Bob.