Ditto, you have a vigor issue, NOT a hatch date issue. Breed for better vigor for a few years, cull super hard and you will see your problem disappear.
It is kind of pointless to even bother worrying about other traits until you get this one fixed.
Start at the incubator BEFORE hatch; Cull developing eggs for vigor- no moving embryo or healthy looking veins at 10 days, then toss them, assume at least a 50% cull rate. Weigh the chicks at hatch and set sensible goals for weight, weigh again at 2, 3, and 6 weeks. Breed enough chicks that you can do a minimum 10% cull at each age. First hatches, will be control groups for taking a baseline of weights-
you cannot move forward unless you know where you are now. Although all of this sounds harsh and labor intensive, if you follow this you *should* very quickly make real progress. There is a likelyhood of a higher percentage of roos w/ this method due to their possibly being larger... but you will have the issue licked in 2 seasons or less and can move on to bigger and better chickens.
**assuming your incubation techniques/equipment are already at optimum**