Detecting dialog: Why is AI not better at this?

Over the past few months, my friend and I have gone deep using the Microsoft and Google AI platforms to try and reliably divide text that is spoken by someone (dialog) from unspoken in a block of text. I would have thought this to be a very straightforward task for an LLM. It is not, and we're still trying to produce reliable results. Even when adding the middle step of "First, replace all opening quotation marks with

and closing quotation …

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