I can echo what Lulu100 said, at least as regards BA. We also were told that there are jobs for everyone as long as they hit the target score....whatever that is. Don't give up! if you really want to do it then stick with it, even if you have to go away for awhile and gain a bit more C/S experience.
My first interview with BA was nearly 15 years ago and i know I wasn't really ready for it....but also they did have more stringent criteria back then such as a language and my Italian wasn't up to scratch. then i applied for United Airlines and got through the first 2 interviews which were bloody tough , only to blow it on the 2:1. I walked into the room and before my bum had touched the chair they said " so , tell us about Gary"
i just blanked...completely. It was awful and from then on it was a downward spiral.
I decided to leave it some time before reapplying. I went out of my way to get jobs I really didn't want such as Barman, waiter, theatre manager, night club manager, anything that would put me right in there with all the horrors of the general public. And it paid off!
I know its frustrating, but if you want it bad enough then you will get it.
Go backover the questions they asked you,answer them in a different way and never rush your answers, don't panic, if you're not sure then say "Can I come back to that one" don't forget that they wan tyou to succeed, they aren't the enemy. Try to recall your body language during the interview...this, I think, was very important and i was told numerous times about it by CC friends. You MUST give the impression that YOU are in control, even if you're shaking like a leaf inside. Just keep your hands lightly in your lap or on your legs with all fingertips just slightly touching each other ( NLP tip..this indicates that you are in touch with yourself).
Also, and this really helped me, I bought a book titled...Brilliant answers to tough interview questions... by Susan Hodgson. Well worth the £9.
And look back through all the threads on here..they were invaluable to me.
Another way to help on the day of the interview is a bit of Hypnotherapy.
It's not for everyone but i'm biased as i trained for 2 years and qualified as a clinical Hypnotherapist!
seriously, sometimes we only need to be in a slightly more relaxed state to allow our real self to shine through. I've helped lots of people with fear of flying and if you are open to it, it can really help interview technique by enhancing your delivery of answers and presentation.
Also, don't forget that you were good enough to actually be invited for an assessment - you were chosen out of many people who applied! You already have a foot in the door!
Don't give up!!!!!!