Now and then, an illustration will not be made, and you will see a short message explaining why. This is a good thing: it means a problem was caught early, usually before you were charged. Here are the reasons you might see and the fix for each.
"Add reference pictures"
The most common one. A page needs a character (or setting, or prop) that has no reference picture yet, so there is nothing to draw them from. Storybench refuses the picture before charging you, so your credits are untouched.
The fix: open the character (or setting or prop) that is missing references, add at least one reference picture (upload or generate and keep one), and illustrate the page again. See build your cast.
"Too many reference pictures"
There is a limit to how many pictures can feed a single illustration. If a page pulls in a lot of characters and props, each with several references, it can add up past that limit.
The fix: trim what the page needs to draw from. You can include fewer characters or props on that page, or reduce the number of reference pictures on the ones involved. A smaller, stronger set of references works better anyway, as covered in reference pictures, explained.
"Not enough credits"
Making an illustration uses credits, and your balance is too low for this one.
The fix: top up your credits, and try again. Remember that building, writing, and exporting are all free: it is only the picture-making that draws on credits. See browsing and costs.
If a description gets flagged
Occasionally a picture cannot be made because the wording tripped a content filter. If this happens, gently rephrase your page text or art instruction and try again. Your credits are not spent when this occurs.
The reassuring part
In every case above, a refused illustration means you were protected from paying for a picture that could not be made properly. Fix the small thing it points to, illustrate again, and you are on your way.