The Machinists union has withdrawn its filing for a representation election among flight attendants at Delta Air Lines, marking the latest setback for organizers at the carrier.

Once the National Mediation Board formally dismisses the union’s application, no filing for a unionization election among the airline’s flight attendants is allowed for one year, according to Atlanta-based Delta. The mediation board governs labor relations at airlines and railroads.

The International Association of Machinists had filed for the election in January, marking the fourth attempt at organizing the Delta flight attendants in the last 13 years. In a message on its campaign website, the union said it filed the cards with the mediation board “as soon as we received what was believed to be a substantial majority of authorization cards” from flight attendants.

But, said Machinists union spokesman James Carlson, “we just recently found out that there were cards submitted with insufficient information or questionable signatures and the decision was made to withdraw the application instead of waiting a lot of time” for a determination from the board. “Because each day that passes with a dismissal… that would just push the process back further.”

The union said it plans to continue its Delta campaign and intends to file for an election once 12 months have passed. Delta’s pilots are the only large organized work group at the Atlanta-based company, making it the least unionized big U.S. airline.