Final answer:
The amendment that wasn't enforced when fugitive slaves were returned is the 13th Amendment.
Step-by-step explanation:
The amendment that wasn't enforced when fugitive slaves were returned is the 13th Amendment. This amendment, ratified at the end of the Civil War, abolished slavery in the United States, making it unconstitutional. However, despite the 13th Amendment, the provision for returning fugitive slaves under the fugitive slave clause of the Constitution was not null and void until the adoption of the 13th Amendment.