Hey, Jean. I think you might not be setting the sustain point. It's the dividing line that marks the placement of your Note Off in the lifespan of a note. Anything before it would be the 'norma'l stages of attack/decay, and anything after it begins the release stage. To set a sustain point, highlight the desired node, and press "S" on the Qwerty keyboard. You should see a reddish-orange vertical line appear. Do this, and everything else will fall into place for you.
One thing that I always did in the beginning was forget to enable the status of the Amp EG to On. When that's the case, the hidden "master" Amp EG takes over, with an instant attack, flat decay/sustain portion, and an immediate release to zero level. I must've done that dozens of times at the start.
I never did get to my promised Nodes & Segments tutorial, but there is an advanced one on the P5 wiki. Maybe you can weed through that and get something out of it:
Multiple AttackDecayRelease.