The word they in paragraph 1, refers to:

Production bottlenecks are generally considered to be temporary blockades to increased output; they can be thrown up anywhere along the course of a production process. Some are easy to identify and to remedy, while others are devilish.

The bottleneck that is easy to cope................................. is stationary. Work-in-process inventory piles..................... quickly behind it; clearly, little is getting through. Its cause is usually also clear – a machine has broken........................ or key workers are absent or demand has simply outstripped the clear, rated capacity of a machine – and the remedy follows easily. Such bottlenecks often occur........................ service operations, causing customer waits.

More subtle are bottlenecks that shift from one part of the process to another or that have no clear cause. Inventories build up in different places and at different times. Such bottlenecks creep up on management and demand more thorough investigation. Perhaps they were detected as flaws in a product’s quality caused inadvertently by one or more workers trying to keep pace with production demands that should not have been placed on them. Or, they may be caused by missing parts. They may be caused by new product startup or changes in the mix of products through the factory. In such cases the remedies are less clear-cut, and some analysis is called for.