I have a bunch of custom field sets used for contributions. Most of them show up as collapsed groups when I edit a single contribution, but one of them does not. Is there a method to show a custom field set on the individual contribution?
To debug this issue, examine the missing custom field (Administer > Custom Data and Screens > Custom Fields) and what exactly it is extending on a contribution entity. Is it for Contributions in general, or does the custom field that is "missing" only apply to a certain financial type potentially?
Also, check if there are any customization that may be not be showing them. If you are an admin, you can probably rule this out.