Chlorine lock doesn't ever really occur in the sense that at some CYA level disinfection stops completely regardless of FC level. At higher CYA levels you simply need to have higher FC levels to maintain the same level of disinfecting, oxidizing and prevention of algae capability. With a lot of CYA in the water, it is possible to slow down the dissolving of slow-dissolving Trichlor (that happened to me when my CYA got over 100).

See this chart which really could continue on at even higher CYA levels having even higher Min FC levels.

The issue is that with higher FC levels at higher CYA levels you are wasting more chlorine because about half of the FC is going to get consumed by sunlight each day and half of a larger number is a larger number. However, as waterbear said, with too low an FC, you simply won't be able to stay on top of your chlorine fast enough. It's a tradeoff between chlorine waste (at higher CYA and FC) vs. how quickly and frequently you are able to replenish the chlorine level. Most people don't want to spend every hour by their pool adding more chlorine.

For SWG pools, there is a completely separate issue as to why higher CYA levels are required and this has to do with SWG cell efficiency, but that's discussed on this thread.

Richard