Seed setting shall depend upon the degree of shock given during the process of emasculation, pollination, bagging etc. There are reports of genetic variation to sustain these shocks but to me it largely depends on the interaction of genotype versus skill. Secondly crop like maize shall also be attribute to inbreeding depression while selfing which is directly related to fitness of genotype in question while crop like rice have better homozygotic balance and lesser inflenced by inbreeding.
2-4 D NAA and Gibberellin can increase fruit set with other practices like flag leaf removal. It is impirtant to synchronize male and female donors but these aspects are in the knowledge bag of breeders. I suggest a visit