It is said that in order to paint well, you must learn to see.
And it's not as easy as you'd think. Most of us "see" - but we don't "observe."
When you are struggling for a likeness, make it a point to observe each side of the face separately.
Never ever assume that one side of a person's face looks like the other side.
Thanks to the miracle of Photoshop, I have "rearranged" Hillary's face, i.e., put right sides with right sides and left sides with left sides.
As you can see, her unique likeness gets completely lost when I do this.
Rule # 2
Even cute little Ryan has an asymmetrical face. And the older a baby gets, the more asymmetrical they become.
I think that asymmetry is one of the things that make us human.
Mother nature reserves symmetry for animals...not people.
Maybe that is why animals are so much easier to draw and "likeness" is seldom such an issue with a portrait of your pooch, goldfish, gerbil or cow.
So if you want to draw or paint people really well, you have to learn to "observe" each side of the face and see what is really there.
Never assume anything is "true" unless you observe it to be so.