Require an artifact to enter the late stage — a proposal link, a signed order form — so the stage cannot run ahead of the paper. Then report stage age and close-date slip count alongside the forecast, which makes the stalled deals visible without a policing exercise.
The behavioural cause is that late stages have no cost to enter and a real cost to leave. Adding an artifact gate changes the incentive without anyone having to argue about a specific deal. Pair it with a written definition of what triggers a deal moving backwards.
Every claim above comes from work LeanScale published. These are the sources.