juggling - ok, take a couple of examples. They are from people I've known, but similar things could happen at any stage of education.
Example one, let's call him Tom. Tom's mum has looked a lot on the web and read up on dyslexia and she notices he seems to fit the diagnosis criteria. Tom gets used to the idea. At 18 Tom goes to university. Now he's finding it quite hard. His essays are getting bad marks. When exams are coming up, it occurs to Tom that he really should be getting extra time during exams like other dyslexic students - he hasn't needed it before but now he's struggling, it'd help. He goes to me, his teacher, and explains he is dyslexic and needs extra time. I notice that he doesn't come up in my records as having a DSA or an assessment we know about, and I say so. An assessment costs 500 pouds and the waiting list is currently three months. Tom fails his first year exams. If he's lucky, he'll get the diagnosis, get some help, and come back next year.
Example two, let's call her Jane. Jane's mum, likewise, reckons she's dyslexic. Jane goes to university, same story, and struggles. Jane asks me for help - she's dyslexic, she thinks she needs some support. We get her an assessment. It comes back saying, well, no. The ed pysch doesn't see anything vaguely like dyslexia.
It is actually more crushing, IME, to be the Jane example than the Tom. It is really cruel to have been told you have something and find it's not so, because 'Jane', through no fault of her own, feels she's been a fraud, and feels she's got 'no excuse' for her difficulties. This is of course not true. Jane may well still need support and help - but she'll have to adjust to the fact that a safety net she thought was there, is not there. And a problem she'd got used to thinking she had, is not the same as the actual problem she has.