I put the slow start down to young birds and one of my cages was over-stocked at the start of the season. Hopefully I'll end up with 13 from two pair.
As for losing nestlings, there is nothing that I can pin point.
Young parents? Maybe.
Bad weather? I don't think so as they were still at brooding age.
Food? Unlikely as they got the same grub all year.
Lack of daylight/feeding? Maybe as it is a dark aviary and birds roost early.
Mystery illness? Not sure.