One likely explanation of course is that we have different conditions, another is that the condition itself expresses in different ways. But for me, to cut short yet another long story I'm about to tell, it's taken 15 years to identify key factors, one every few years, through chance breakthroughs really, and only very recently have I learned enough of the multiple patterns to separate causes and effects with confidence. Usual caveats apply: everything here is of course only my experience and will only work for you if you have something the same or similar to what I've got, which is multiple atypical immune sensitivities triggering multiple (CFS) symptoms, experienced as relentless fatigue when in constant contact with something I'm sensitive to - as I was for many years when unknown to me, my clothes and bedding were constantly wearing down my energy to the point of exhaustion.
In my case I feel - currently - largely recovered in the sense that, after a decade of gradual but confusing improvements and relapses with perhaps a general upward trend, trying all kinds of things and never being quite sure what worked, now I've reached a stage where by avoiding almost all physical activity, striving to avoid and filter everything from washing powders to perfumes, eating a very restricted diet, etc etc etc, that only now when I'm identifying the last few triggers - maybe one a year - can I really feel confident that I really am noticing cause and effect. (Oh and by the way I do still regularly relapse for a day or two at a time and have to detox thoroughly, that's the way it goes at the moment).
Again, it's only me, but the explanation of how I personally progressed from frustrating confusion over the patterns - just as you describe Jenny, for years and years - is: painstaking experimentation, trial and error, expensive tests usually pointless, eventually leading to one or two key breakthrough observations (perhaps after a few blind strokes of luck to just take the right supplements or medicines). I suppose one key thing was a year when I said: right: devote pretty much every spare minute to health for one year: try everything on this huge list of things, try everything feasible-sounding that people have suggested, from food to environment control to meditation and supplementation - all at the same time. I went off gluten then too (and btw: smoothies, rice or corn pasta, potatoes and veg are my replacement for bread btw, not the gluten-replacement sort of stuff really although I do still steer well clear of gluten most of the time and that showed fairly strong on the test).
After a period when I had eliminated so many things that I was probably only getting 3 or 4 triggers regularly (plus the standard background triggers of mild colds/flus and events involving inflammation), at that point it began to be possible to have correct suspicions about associations between specific foods and the 12-24-hour delayed on/off switch of digestive/sensitivity/fatigue symptoms. Before I had the Allergy UK test I had just got the suspicion of egg as a trigger and was starting to test by elimination: so no surprise when egg was one of the 3 new triggers identified by the test and the other 2 proved accurate also (soya explaining SO much!).
In summary, the way I think about it, right or wrong, is this: if there are scores of potential triggers, some acting immediately and some delayed by hours, then even after you've figured out all the secondary and tertiary symptoms, you're not going to easily observe the primary triggers in action until you've already worked out or guessed most of them, and although you get tantalising clues before that point, there's so much background noise that all your theories keep apparently falling down. Don't lose hope and trust your feelings: don't be put off by confusing observations, accept that it really is as complex as it seems. There's so much to complicate the picture: some or all of food sensitivities, invisible environmental triggers, hormonal triggers (making the process I've described dramatically harder for women of course), viruses, inflammatory events, and more I'm sure.
I hope this isn't all just frustrating to read, I know it would have been for me a few years ago so I'm sorry if so. All I can do is share what worked/works for me - for the time being at least - and hope it's helpful to somebody. :Retro smile: