The custom subject heading feature uses exact matching as a part of the customization process. It is possible that a spreadsheet software like Excel could automatically be set up to make formatting changes like eliminating what it sees as un-needed spaces. For this reason, attention to detail matters, as a small difference or a typo can cause a subject to not match correctly.
Below is an example of a subject that did not match correctly due to spaces being omitted in the file.
MARC subject: Blind -- Juvenile fiction
Subject in the file: Blind--Juvenile fiction
Notice the spaces around the hyphens in the MARC entry, this would cause the two bolded lines to not be a match.
Create locally preferred subjects for display and search expansion