Try techniques like the 5 whys and the circles and the soup.
These keep the main thing the main thing. If the conversation starts to wander or if the team isn't getting deep enough, know when and how to facilitate the team back onto something tangible they can impact. The goal is to come out of the meeting with a viable experiment to try for the next iteration. To get to that, they need to remain focused on what they can control and what they believe will impact the issues they have identified. The root cause.
Consider fun ways to identify the root cause (pin the tail on the donkey, fishbone diagrams) and to keep it front of mind (large wanted poster, bulls eye, giant sticky note).