“What was your least favorite part of the session?”
“What do you want to see happen next?”
Of course, you don’t have to ask these questions. But if you don’t ask for feedback, or the feedback you get isn’t helpful, these questions tend to get to the heart of the matter.
Vague questions, like “So, how was the session?” or “So, what did you think?” get vague answers, like, “Good” or “Fine.” (Just as vague questions like “How are you?” get a vague answer of “Good.”)
You want to know what went right. What did the players respond to? What excited them, inspired them?
You also want to know what went wrong. You think you know, but the answer might surprise you.
And you want to plan for next session. Let the players tell you.
Protip: If one of your players stays quiet, ask them directly. Show them that their feedback matters to you, too.