On going chest problems

[Coffeemonster1]

Our lg is 18 months and so far has had 4 lots of double pneumonia, and 4 double chest infections, which have required 6 separate hospital stays for anywhere between 2 days up to 2 weeks of oxygen and IV antibiotics. She gets discharged and after a week or a few if were lucky, the cough, temperature and chesty rattling sound starts again and progressively gets worse until she requires hospital treatment again.
She struggles to gain weight, which she has high calorie milk for and occasionally an NG tube put down to to up her intake both day and night, she hasn't had a clear X-ray (she has one every 6-8 weeks) in 11 months, she always sounds rattly and coughs to the point she vomits at least 4/5 times a week.
She has been tested for cystic fibrosis, immune disorders, cancers etc and had swallow tests etc and under Oxford who plan to do a ct scan and endoscopy at some point but consultant keeps saying it could just be bad luck???
Now I could understand if she was a generally unwell baby but she goes to nursery and has done since I went back to work at 7 months old and has never caught a sick bug, ear or throat infection, conjunctivitis or anything other than chest problems, which if she was just picking up ALL the common baby things it would be a range of virus's etc ?. Why wouldn't antibiotics cure her infection if that's all it was?
Her bloods were done in hospital, specifically infection levels which were 1400 when admitted and after 2 weeks of IV antibiotics they were still 1200 which shows the antibiotics obviously aren't curing or getting to the bottom of things.
Has anyone experienced this and got to the bottom of it? Our local hospital have said apart from treating the symptoms, there isn't anything else test wise they can do, but they do all agree that something is being missed,but it's down to Oxford to continue the search.
Any advice, help or suggestions are very much welcome :) xxx