If everything else is in place (proper amount of protein, free of illness/disease, eggs not slightly freezing then thawing in the nest boxes), then it may be nothing more than genetics. Here's something I found online (these are 6 conclusion drawn regarding runny egg whites):
1. Occasional eggs with spreading (runny) whites are observed originating from apparently normal flocks. 2. The runny eggs tend to be laid by the same hens. 3. The existence of runny eggs has nothing to do with freshness; it can be observed in newly laid eggs. 4. The albumen height and Haugh unit rating is not different between runny and normal eggs. 5. There are differences in biochemical composition between normal and runny eggs. 6. There appears to be a genetic effect on the incidence of runny eggs, suggesting that selection might reduce the incidence.