Sadly, at the moment there is dramatic oversupply of candidates, so the RAF can afford to be extremely choosy over who gets in and who doesn't. This isn't just the RAF, if you look at the processes by which the Army and Navy take recruits, these have changed dramatically too. In years gone by, it was a case of turning up, having a very cursory medical (literally counting limbs) and then you'd start basic training, who would weed out about 10% of candidates in the first weeks for various previously undiagnosed health matters, and then spend a lot of time getting all candidates up to the required fitness levels etc.
Nowadays, the armed forces are full up. So why do all that hard work? They just take the ones that are already fit, and make them pass the medicals before joining. Sadly, if there are twenty people wanting to fill five places, and ten of them have had hayfever (however mild), it costs the RAF nothing to exclude them, as they still have loads of candidates to choose from.
Personally, I put the blame for a lot of the RAF disappointment at the door of the air cadet organisation. They are as bad as anyone for encouraging candidates to set their hearts on careers when they have no chance of fulfilling them.