thay will throw flights and it is not good for the birds to stop and start moult once thay have thrown a flight you might as well let them carry on, the idea of darkness is to stop them throwing flights and just go throw body moult.
i first used darkness 1986 and over the years i have tried all different start dates and to me i have found best to put them on when weaned winning or ru up yb ave most years, but you have not said how old your ybs are now.
i use layers pellets evey year, but got some bos pellets free with my breed & wean corn & the birds have gone mad on them ybs in nests look 1st class would recomend to anyone.
i started when i was 11 yrs old no one in family had them before me,i brought my first two ybs home from a fancier i met on my paper round, as my dad had just passed away i dont think my mam had the heart to make me take them back, now had them for48 yrs could not be without them.
i have to agree ybs never settled in the first place will do this every time, thay have only known a confind area once out in the big open sky thay will panick and be miles away, best bet is to keep them in.
the reason it was lost of loft is it was blind as whites with pink eyes normaly are, also can happen with breeds of white dogs. oldyellows bred them a few years back lost off loft,