Hope this helps:
"Soft-shelled eggs
The first pullet egg may be soft-shelled until her system gets into its stride. If it continues, make sure that the birds are getting a balanced diet such as that provided by a commercial free-range or organic layer�s ration. Such feeds will usually contain calcium and phosphorus in the right ratio (around 3.5-4% calcium to 0.3% phosphorus). Providing a little crushed oyster-shell or calcified seaweed will ensure that any deficiency is rectified, for the birds will not take more than they require.
A shock can also make a hen lay a soft-shelled egg. My own observations are that if a flock is caught in a sudden shower of rain (for they are sometimes too dim to run for shelter), a few soft-shelled eggs are often produced the next day, but by the following day, they�re back to normal.
It is when soft-shelled eggs or misshapen ones are produced regularly that there need be a cause for concern. Veterinary advice should be sought. Conditions that adversely affect eggs include Newcastle disease (a notifiable disease to the authorities) and Infectious bronchitis, but there would be disease symptoms showing in the birds themselves if either of these was present. Hybrids are normally vaccinated against them.
Egg drop syndrome (EDS) is also a viral infection that results in a reduced number of eggs, as well as an increased number of pale-shelled eggs. Birds do recover from it but egg production may not get back to its previous level and there may still be a proportion of deformed ones produced. It can be vaccinated against. "