Denver-based Frontier Airlines is still near the bottom of a monthly customer satisfaction survey of the airline industry.

The U.S. Department of Transportation follows on-time performance and customer complaints among the top 13 U.S. airlines. Frontier is number 12 in both categories in the latest report, which measures July.

Customer complaints have been up since Frontier made itself into a so-called “ultra low cost” carrier in 2014, cutting basic fares, but charging extra for services other airlines include in the ticket price. The rising complaints led to the resignation of Frontier’s chief executive earlier this year.