What can stop an image from being made.
Image prompts are checked before you spend any credits, so a refused prompt costs nothing.
If prompts keep getting refused, image generation pauses for a while, and the pause grows longer if it keeps happening.
What isn't allowed is listed in section 6 of the Terms of Service, Prohibited Content & Conduct.
An image model can occasionally turn down a request on its own. GPT Image is the strictest. When that happens, your credits are refunded and you'll see a message saying so. Rewording the prompt, or switching your world's art model, usually helps.
At busy times you may see "The server is busy. Please try again shortly." Nothing was charged. Try again in a moment.
Any image that fails, for whatever reason, is refunded automatically.