That being said, I have also realized (especially this year) that the students stuck on low partials typically have their lips rolled way too far out, almost like they are puckering, and they probably need to pull it in some at the center. It's a fine balance between the right amount of rolling in and too much tension in the center, but it's an important thing to note.
Last thing, if you have a mouthpiece visualizer, use it. The lightbulb really turned on for my lower kids when I demonstrated what my face looked like with the visualizer and then showed them their embouchure with a mirror. They noticed how much more pink they had and it almost immediately fixed a few of them.