Uniqlo is the latest brand to face a headache over cotton from China's Xinjiang region.
Customs documents show that the U.S. blocked imports of some shirts back in January.
That was over concerns they violated a ban on cotton products from Xinjiang, where there have been reports of forced labor.
A document dated May 10 shows that one shipment was impounded at the Port of Los Angeles on January 5.
The document says a protest filed by Uniqlo parent company Fast Retailing was denied.
There was no response Wednesday (May 19) to a request for comment from the Japanese company.
A China foreign ministry spokesman said there was no forced labor, and the U.S. was using "bullying" tactics.
He called on affected companies to stand up to what he called Washington's "irrational acts".