Hi Garry
Some girls do seem to lay more of these soft shelled eggs than others
They go to move them after laying and obviously their beaks break the shell - so they eat it
Laying these type of eggs also seems to take alot out of them physically
Have they got access to grit & oyster shell at all times?
You could try adding some Limestone Powder to their food to give them some extra calcium (needed to make the shell) and see if that helps.
You can get the powder from most horsey type feed shops.
Sarah