The most common reason prioritisation workshops fail is that the question is too vague. "What should we focus on?" sounds like a reasonable prompt, but it's so broad that everyone in the room is answering a different question. The product manager is thinking about features. The engineering lead is thinking about tech debt. The designer is thinking about user research. Without a shared frame, the discussion becomes a polite contest between competing worldviews, and the outcome is whatever the facilitator manages to synthesise from the noise.
The second failure mode is domination by the loudest voice. Workshops are supposed to be collaborative, but in practice they often devolve into a dialogue between the two or three most assertive people in the room. Everyone else checks out, doodles in their notebook, and waits for it to end. Their input is never captured, which means the workshop output represents a fraction of the group's actual knowledge and preferences. The quiet person in the corner who has the most customer contact might have the best insight in the room — and nobody will ever know.
The third failure is confusing discussion with decision. The group talks for an hour, the facilitator writes up a summary, and everyone nods because they're tired of the conversation. But nodding is not committing. If people don't feel genuine ownership of the output — if they feel it was imposed by the facilitator's interpretation rather than produced by a fair process — they'll quietly ignore it. The list goes into a slide deck, the slide deck goes into a shared drive, and nothing changes. Three months later, someone suggests having a prioritisation workshop.