Breathing difficulties :can Montelukast (singulair) stop working

Hello, first time poster here who spent all day yesterday reading all the posts to try and find help.

Lots and lots of similar posts but I'm compelled to write my own in the hope that someone, somewhere will be able to help.

My daugter (now 6), had no medical problems until she had an attack of croup in January following her 3rd birthday. Over the next 2 months she had a further 2 attacks. All 3 attacks were approx 6 weeks apart. The last one was in the March.

During May of that year she started having difficulty breathing. Even walking out to the car left her breathless and exertion on any level was torture. This continued until September and suddenly ceased. No medication or tests were offered/given by the GP.

Everything was fine until the following May when it all started up again and the GP prescribed a reliever inhaler which had no effect. Between May and September she was given preventer inhalers in various strengths, medications and dosages but none of them worked. The GP did say though that there was signs of allergy in her nose and eyes so also put her on Cetirizine tablets - one a day. There was no improvement.

From September to the following May (which is May this year) ....not a problem.

This May it all started up again and we were at our wits end because nothing seemed to be working. At this point no allergy tests had been done so I requested a referral to see a specialist. While we were waiting for the appointment she was given Montelukast (Singulair in the US) around June time and overnight the difference was remarkable! They were like the wonder pill!

We were so relieved. I should point out here that there still hasn't been an allergy test.

They worked perfect until about 3 weeks ago (end of October so they worked for about 5 months) and they suddenly stopped working. We are devestated because 1) they've stopped working and 2) my daughter is back to her breathless state and it's gone past September.

I've always maintianed that I didn't think it was asthma because other than the breathlessness (which is daily and all day) she doesn't seem to have any of the other usual sypmtoms. She doesn't wheeze, she doesn't cough, she's fine at night and none of the asthma medications she's been on have made a difference.

This week the asthma nurse has said that finally there's agreement and it's not asthma but she is convinced it's hyperventilating. I'm at my wits end! There still haven't been any allergy tests (and even to my untrained eye the inside of her nose is really inflamed and red and puffy) but instead of looking for the allergens, she's trying to teach new breathing techniques as a means to control it (mainly nose breathing which is causing distress and having little effect because of the swelling).

Her next solution is to have my daughter in for a few days to run exercise tests and observe overnights because she thinks it may be cardiac.

I think I'm slowly going mad. Cardiac? There's no chest pain, no hypertension, no family history, no previous myocardial infaction and no abnormal ECG. Why would that be appropriate before an allergy test.?

A 3 day stay in hospital for these tests or an allergy test.

When I try to point this out I'm getting her annoyed and she twists it back to me and why wouldn't I want to go for the full battery of tests.

I think it's overkill and unnessasary but she's making me feel like a bad mum. Allergy has been mentioned since day 1 and I feel that if we knew what they were, we could take any steps needed to put triggers at a minimum to try and help.

This is really long and rambling, I know. It's because I'm distraught with worry and no-one is listening to me. I'm also under pressure because on her bad days my daughter is missing school because she's breathless by the time she gets there and there's nothing that will put the air back in her, other than not moving. At school that's not possible. Everyone seems to spend more time fretting about the school absence than finding out how to stop the cause of the absence and I feel like I'm dammed if I do and dammed if I don't.

Does anyone know if the Montelukast (singulair) could just stop working?
Does anyone have anything similar to these symptoms?

Just a recap.

Breathless May to September (until this year)
Only med to work was Montelukast, which has stopped working after 5 months.
Nothing we know of has changed in activity, location, environment.

I'm asking for guidance because the advice I'm being given goes against what my gut feeling is telling me and when I try to ask for things to be done in a logical path, I'm being met with hostility.

I should point out that I'm in the UK and the service is provided by the NHS which has notoriously long waiting times and who probably operate in a different manner to the healthcare sevices in other countries.
But all I want is to find out a) what's causing my 6 year old daughters breathing problmes and b) what to do to try and remedy them.

I'm sorry this is so long. But PLEASE if you have ANY advice or links or suggestions for anything that I've written, no matter how small, please post.

Thank you.

On a side note, we have been given an appointment to go in (for what I assume will be the long awaited allergy tests) but I'd like to ask - what is the proceedure for afterwards? If the tests come back with allergies, what would normally happen? Would there be a histamine blocker precribed and time given to see if it works? I ask because I'm not familiar with allergy and I feel like I'm just 1 little cog in the big healthcare machine and all the other, professional cogs are dismissing my input. I'd like to know what I'm talking about when I go for the appointment.