Egg problem

One of my hens keeps laying chalky eggs or eggs with soft ends or very occasionally no shell ! I'm assuming its calcium deficiency but what do I give em to solve the problem ? They are fed mainly ad lib on layers pellets, mixed grain in the evening 1 handful between 3 hens, grit always available, and needless to say copious quantities of water inside and out of the pen, all food stuffs are fed inside.
Thanks

Cheers Karl knew there was something I was missing tried giving em baked and crushed egg shell that has helped a bit.

If you are giving them crushed egg shell it must be baked in the oven for 20 mins to kill bacteria, as muntjac pointed out it can cause infections.  

If crushed oyster shell doesn't completely work then the hens would benifit from some vitamin D. Short term cod liver oil would be OK but not long term as too much fat in the diet prevents calcium uptake and your back to square one.

There is a product called Shellstim containing vitamin D which is designed for laying hens. Just add to their water until the problem resolves.  Make sure the feed you are using is in date as it loses vitamin content quite quickly after the use by date.

also may help to grate some cuttle fish onto feed.

My Laydees all stopped laying about a month ago and have only produced a few yolks and soft shells since. The bantams stopped laying just before Christmas but started again last week and are now back to full production.
What is the chickens' problem please?

if the are laying soft eggs the answer is simple gert some oestra down em and some poultry grit , bit obvios if they are actually laying yolks they havent stopped laying just struggling to cover them     . if they are laying hard eggs only once a week or so they are taking a rest