The most valuable person on any software project is usually someone who doesn't write code. They're the person who's been doing the job for ten years and knows every edge case, every exception, every "oh but on Fridays we do it differently."

They don't call themselves a domain expert. They don't write requirements documents. They just know things. And if you don't find them and talk to them, you'll build software that handles the happy path perfectly and breaks on every real-world exception.

Keep asking about exceptions. "Is it always like this? Always? Literally every single time? Can you think of a time when it wasn't like this?" That's where the real requirements live.