PUEBLA, Mexico—Workers at Volkswagen’s assembly plant here will be asked to vote again on whether to approve their union’s deal for 9 percent raises after initially rejecting the agreement.

The Independent Union of Automotive Workers (SITIAVW) reached a deal with the company for what would have been the biggest raise in the Mexican automotive industry in recent years, covering some 7,000 people, but the plan failed to pass a worker’s vote needed for approval.

SITIAVW, one of Mexico’s strongest independent unions, initially sought a raise above 15 percent, citing inflation. Last year, it negotiated a 5.5 percent increase with Volkswagen and in July settled on a 9 percent hike for the next year-long pay cycle.

Union salaries at the Puebla plant range from about $15 to $48 a day.

Mexico’s Federal Center for Labor Conciliation and Registration has instructed the union to redo the vote in an effort to ensure higher turnout. In the initial vote on Aug. 5, about 70 percent of eligible workers cast ballots. The “No” votes won by 338 ballots.