What does a freelance designer have to do to get PAID??

In short, I have 12 years industry experience, an impressive portfolio and skills. My problem is that I meet with prospective clients who hire me based on my portfolio, they send through a brief I send through a quote, which gets accepted, I do the work asap as they need it yesterday - I then wait 3-4 months for them to proof and then when I ask them to pay me, I get asked to give them a breakdown of all of the work that was involved etc etc..........What do I do???