For two years every portrait I finished had the same problem. The face looked like it was made of smooth plastic. People in the community kept saying to study light and shadow, which was not wrong advice, but it was not specific enough to fix what I was doing.
The actual problem was layer blending
I was painting all my shadows on a single Normal layer directly over the base color. That method flattens form because it treats shadow as a color replacement rather than a light interaction. The moment I switched to Multiply layers for shadows and experimented with Overlay for highlights, the depth appeared almost immediately.
A specific example from one piece
I had a portrait of an older woman with strong cheekbone definition. On Normal layers her face read as a flat diagram. I rebuilt the shadow structure on Multiply at around 40 percent opacity, added a second Multiply pass at 20 percent for the deepest areas, then used a soft Overlay layer for the reflected light on her jaw. The face suddenly had weight.
This is the kind of specific decision point that tutorials skip over because they assume you already understand blending modes conceptually. Understanding them conceptually and knowing when to use which one are two different things.
Gardening and Landscape Design uses a similar layering logic. You do not add all nutrients at once. You layer inputs based on what the soil already holds.
If your portraits feel flat, check your layer structure before assuming your drawing is wrong.