You have tried to solve an issue with the airline directly, but have come to a dead end. Filing a complaint with the Department of Transportation’s Office of Aviation Enforcement and Proceedings most likely would help and is very easy to do online.
Normally, I have been able to solve an issue that I have had an airline by going through the usual customer service channels. Some airlines take their time to reply where others are very good at solving issues that have arisen during the travels promptly. I have faced issues that have required me to contact DOT’s Office of Aviation Enforcement and Proceedings only couple of times.
Based on my personal experience, airlines tend to take the complaints that come through DOT more seriously than from their usual customer service channels, and resolve them expediently. My advice is to first contact the airline in question and give them a chance to make it right. If they don’t, then file a complaint with the DOT.
The complaint process is very straightforward. You can either email your complaint to the DOT or file the web form at http://airconsumer.dot.gov/escomplaint/es.cfm. To save DOT’s time it is probably better to fill out the web form that helps them to capture all the necessarily data than calling them. Note that DOT doesn’t act based on an individual complaint, but does forward them to the airline in question for further review. The DOT produces a monthly Air Travel Consumer Report that includes the number of complaints that each individual airlines has received through the DOT.
Also, the Air Travel Consumer Report is worth reading once in a while. It includes important data about the performance of various airlines and statistics about involuntarily denied boarding’s, flight delays, mishandled baggage, number of complaints received per number on enplanements etc. For example based on the last data, United received more than ten times the number of complaints than Southwest per million enplanements.