According to the latest data released Thursday by the U.S. Department of Transportation, flyers filed 15,770 complaints, compared with 12,348 filed in the same January to September window last year. Flight delays, cancellations, and misconnections were the most frequently filed complaints, with baggage-related gripes, boarding, and customer service issues next in line.

The rise in complaints comes despite a drop across the board for a number of key passenger metrics. September’s on-time arrival rate was 86.5%, up from the 81.1% rate in September last year, and above the month’s average of 82.7% over the last 20 years.

The rate of mishandled baggage also dropped to 3.31 bags for every 1,000 passengers for this year’s January to September period, down from 3.70 bags per 1,000 passengers in the same period last year. The rate of passengers who were involuntarily denied boarding due to overbooking—also known as “bumping”—was at 0.77 per 10,000 passengers, down from the rate of 1.04 posted during the first nine months of 2014.

A look back at annual data available at the mid-year point shows that the pace of complaints has been increasing over recent years:

While this represents a fraction of the total number of travelers—there were 1,351 complaints filed against U.S. airlines out of 55.8 million passengers this year, for example—one passenger advocate suggested that complaints may be up because airlines are cutting down on their free amenities and hiking ancillary fees, while economy classes are packing more people in at the cost of comfort.

“I don’t think the flight experience has improved,” Kate Hanni, founder of Flyersrights.org, told the Los Angeles Times. “People are stressed from the time they get to the airport and get through security and get on a plane with very tight seats.”