Personally all of my issues have been with shipped eggs. I have not lost any of the lavs I have hatched form my eggs other than one (freak accident with a feeder). I breed to splits to improve type. For me the type is more important than the color, so I breed to the best black or split I can find. If it means less pure lav chick for a while I am fine with that. I am breeding for the long run and myself, not production or to sell chicks. All of my pens have lav roos with black or split hens. Since I do not have a black roo at the moment I have my lav silkie and lav showgirl hens with my porcelain cock until I can locate a nice black silkie roo.
I may be having such good luck due to using splits and showgirls which may diversify the genetics of my lavs. I have not done much lav to lav at all since the type is not where I want it on all of them yet. I am hoping to produce lavs that can compete with the whites and blacks next year.
A few things I do other than flock raiser rather than layer feed that seem to work for me are:
PlyViSol in chicks water for the first 2-3 days
Medicated chick started crumbles only for the first month
Leave the chicks in the incubator for 24-48 hours after hatching to recuperate
If a chick seems weak at any time I offer PolyVisol in the water again, and sometimes growgel if they are not eating (have not had to do the growgel since I started supplementing the water with PolyViSol.
I am not claiming to be an expert or anything, these are just a few things that seem to work for me and my birds. GOOD LUCK!!