To get to the bottom of the problem you'll have to go back to when they threw their first flight. If they threw it whilst the loft was in the so called darkening stage (pre 21st June) then that would suggest the loft wasn't darkened down enough. If they held a full wing up to the 21st June then started to throw flights like confetti then I believe it to be a feeding issue, perhaps fed a feed too rich in fat/oil based seeds. I race my darkness youngsters paired which I believe helps suppress the moult.