queen rearing success percentage

Working in a commercial apiary and have just ventured into queen rearing on a small scale (70 nucs/week) to meet our needs for pollination and honey hives. We have 210 nucs split into a 3 week rotation and we generally run between 65-80 percent success on a weekly basis. The majority of failures(~70%) have no queen, most of the rest have virgins and we get the odd laying worker. This seems like an awful lot of queens getting lost, eaten, etc. on mating flights, could hive identity/drift be an issue? The failures are not randomly distributed through the yard which is puzzling-- This Week I started with 12 successes in a row, then 3/15, then 21/23, then 1/8, then 12/12 as I moved across the yard. Similar patchiness of success in other yards. Any ideas? The weather seems to be the biggest source of variation on a week to week basis, but the clustering of failures in the yards has me stumped. As its a new part of our operation there is heaps to learn-- so I guess I'm asking what sort of success rates can be expected from queen rearing-- and more importantly how do you get there.

dw