As a former manager of an australian airline contact centre, I understand what you are saying. The hardest thing for agents to deal with is continually being bombarded by negative customers. Unrealistic sales targets can also be a complete demotivator. Remember that everyones needs and expectations are different.
I found that constantly talking to them about adapting to the customer such as holiday customer relaxed mode of conversation and business customers a professional business conversation always helped with the customer.
Complaints in the industry are inevitable and I always tried to have my teams remember the romance of travel as people still see it as important, but not normally romantic. That bygone era had people just happy to get on a plane even if it was late, it was exclusive.
People get passionate and complain when the issue means most to them. A holiday or missed meetings are really bad for the customer, but they forget that life is important. Ask your staff who are an issue to have a work shop to express how they would feel if it were them in the customers shoes. How would they respond. What would their expectations be and how would they responsed . Also ask them what would be their response if they were not employed in the aviation industry.
Passengers dont value safety when pressed for time, but if your survey them they would all say it was vitaly important. Educating the customer is always hard to acheive.
Hope this helps in some way.