An American Airlines Boeing 777-200 aircraftNicolas Economou | NurPhoto | Getty ImagesAmerican Airlines canceled more flights than any other major U.S. carrier and struggled with persistent delays in July, according to new government data. American has blamed the grounded Boeing 737 Max, a bitter labor dispute with its mechanics and summer storms for the mounting disruptions.Fort Worth, Texas-based American canceled 6,529 flights in July, about 3.5% of its schedule, the Department of Transportation said Thursday.”It was a tough summer for us,” CFO Derek Kerr said Wednesday at the Skift Global Forum, a travel industry conference in New York. “We’re focusing on next year.”A federal court last month issued a permanent injunction against American’s mechanics unions, which the airline alleged were engaged in an illegal work slowdown to disrupt operations and win leverage in contract negotiations. The unions have denied American’s allegations, but the airline cites tensions as contributing to hundreds of flight delays.American and the mechanics unions resumed contracts talks this week.What’s more, delays only compound. The impact of out-of-service aircraft early in the morning “ripples through the whole day,” leading to more cancellations and delays, Kerr said.American’s executives are under pressure to improve operations as the carrier’s stock — already down 11% this year — lags its competitors. Southwest is up 19%, United is up 7% and Delta is up 18% in 2019.Customer service agents have been calling some passengers affected by the disruptions to apologize and offer compensation, such as frequent flyer miles.American is not the only airline grappling with the grounding of the Boeing 737 Max, which has led to thousands of flight cancellations during the peak summer travel season, or with bad weather.United had the second-highest number of cancellations in July, equal to 2.5% of its schedule. Four stormy days in the month led to more than two-thirds of the carrier’s canceled flights, said United spokesman Charlie Hobart.”These are always challenging months,” he said.American’s on-time arrival rate, though, improved in July to 74.9% from 70.4%, ranking it sixth among airlines for the month, but seventh for the year.Budget carrier Frontier Airlines came in 10th place for on-time arrivals, a rank it’s held since May, with a rate of 71.1%. JetBlue and United ranked ninth and eighth, respectively, the DOT data show.