A manufacturer's on-time delivery rate has dropped from 95% to 80%. How would you find out why?

Two questions before I start. How is on-time defined, is it against the date we promised or the date the customer asked for? And when did the drop start? I'd follow an order from start to finish and find where the time is being lost. An order goes through four stages. One is the promise itself. Did sales start quoting shorter lead times than the plant can meet? Two is materials. Are parts from suppliers arriving when they should? Three is production. That covers machines, labor and scheduling. Four is shipping. Maybe the trucks are late. I'd check how long finished goods wait before they leave. Before going through those, I'd cut the late orders by product, plant and customer. That should narrow it down a lot. Can I see the late orders?

Two questions before I start. How is on-time defined, is it against the date we promised or the date the customer asked for? And when did the drop start? I'll assume it's against our promised date, and that it slid over about six months. I'd follow an order from start to finish and find where the time is being lost. An order goes through four stages. One is the promise itself. Did sales start quoting shorter lead times than the plant can meet? If quotes went from four weeks to three to win orders, deliveries show up as late with nothing wrong in the plant. Two is materials. Are parts from suppliers arriving when they should? One late component can hold up a whole order, so I'd check supplier delivery for the same months. Three is production. That covers machines, labor and scheduling. I'd look at downtime, scrap and changeovers, and whether volume or the number of product variants went up. Four is shipping. Maybe the trucks are late. I'd check how long finished goods wait before they leave. Before going through those, I'd cut the late orders by product, plant and customer. A fall from 95 to 80 is rarely spread evenly. If one product line or one plant explains most of it, that tells us which stage to open first. My hypothesis is that it's upstream of the factory, either tighter promises or a supplier, because a plant doesn't usually lose 15 points in six months on its own. First I'd ask for late orders with a timestamp at each stage. Do we have that?