Too many publishers think those who supply creative content are fair game. Many think that they do not need to pay agreed fees once they have the material in their possession.
Often I get requests for material for no charge in return for a credit. I tell them the only credit I need is in my bank account. If someone reneges on a deal I have no hesitation in pursuing them in whatever way is open to me including the application of the oxygen of publicity.
Never be frightened to ask for what is yours. And do not be dissuaded by those who tell you it was reward enough to see your material published.
I was prompted to write this after reading John Liddiards post. Its not an uncommon story.