At 18½, managing to even attend assessment days for Qantaslink, Virgin, Jetstar and Tigerair is a feat in itself. Some people go many years without even having an audience with a single airline, let alone 4. I personally know crew who have spent the better part of 20 years trying to become a flight attendant, and only have recently reached their goal.
I'm not a recruiter, so I can't speculate with any certainty. But in my opinion, your daughter is just starting to become competitive with her age. Some airlines have recently been trying to replace all the 'oldies' with fresh out of school 18-year-olds, but the attrition rate during training and in their first year has been quite high, so I think more and more airlines are looking at candidates that can demonstrate a reasonable amount of life experience (19 and beyond), and for candidates that are willing to commit to the job.
Your daughter seems to have a considerable amount of experience for her age, and if she performed in the AD's exactly as you've stated, I can't really give a reason as to why she hasn't progressed with any airline. Like Mn64 said, it's just the nature of the industry. You just have to make sure she doesn't give up.
If I had to give a reason, a recruiter would likely look at her resume and besides her work experience will only see the Cert 3 as her only qualification. (besides the necessaries of RSA, Passport, etc). And I would assume other candidates with more certs, diplomas, and degrees would be favoured over her. I would recommend the Aviation Australia cabin crew course. Not only is the Cert II Aviation well regarded from some airlines, it proves that your daughter has the stuff to commit to and pass an airline ground school. AA also have agreements with some of the regional airlines that place AA grads in a preferred recruitment pool, and that's not only a great way to not only get into the industry but to get some varied experience in it as well.
In any case, I wish her the best of luck, and if you have any more questions don't be afraid to ask.