A focused view on why inventory builds between processes and what that usually says about control and flow.
High WIP is rarely caused by one single failure. It's usually the visible symptom of several smaller imbalances compounding across the value stream: uneven process speeds, batch sizes that don't match demand, and scheduling that pushes work forward rather than pulling it through.
When one process runs faster than the one after it, inventory has nowhere to go but to queue. The same happens when batch sizes are set by convenience rather than by what the next process can absorb, or when changeovers are slow enough that operators batch up work to avoid them.
High WIP also hides problems. Excess inventory between processes masks quality issues, absorbs the impact of breakdowns, and makes it harder to see where flow is actually breaking down - which is part of why it tends to grow quietly over time rather than appear all at once.
Reducing it sustainably means rebalancing capacity across the line, shrinking batch sizes where changeovers allow, and introducing pull-based scheduling so work only moves forward when the next step is ready for it.