Being in the US and not knowing what may be different about chicken feeds in the UK and Australia, I would say I doubt it’s the feed. I’ve never heard of specialty breeder feed. I just feed my flock layer pellets (yes, the roosters too) And my eggs hatch just fine.
I don’t know anything about vertically passed illnesses. Again, maybe that’s something more prevalent locally.
What I do know is that 6 eggs is too low of a test number to be able to reach any kind of conclusion about fertility, hatch rate, illness or even genetics. It’s just too small of a sample size. Is there any way you or someone you know could set a batch of 20-40 eggs? 3-4 lost chicks from a batch of of 6 is a high percentage but could just be a coincidence. Those same 3-4 lost chicks in a batch of 30 eggs is not that big a deal. Now if you lose 50% of the larger group, then that for sure raises questions.
In my experience, most of the time when there are low hatch rates it is usually due to hatching conditions. Some incubators work better than others. I’ve also seen where the conditions in the room that the incubator is in change over the course of the season, affecting hatch rates. If you hatch your eggs and they do fine and then other people try to hatch your eggs and they have failures, it’s the other people, not the eggs.
Driving in a car can affect the hatchability of the eggs. It depends on how they were packed, how the buyer handles them, road conditions, distance travelled, whether they get left in a hot car while folks stop for a meal, etc. Lots of variables. Selling hatching eggs is risky because buyers tend to have high expectations and blame you for any problems encountered.
I hope your further test hatches work out well for you. If so, you may want to stick to selling live chicks instead of hatching eggs.
