another soft shell question

I have two young hens. One seems to be laying normal hard shell eggs every day. The other for the last two weeks seems to be laying soft or very soft egs, often more than one at a time and often squished. Is it because there is no hard shell the hen can't lay the egg till perhaps the pressure of futher eggs force them out?
The hens have free range of the garden most of the day. I feed good quality pellets, little extras but they eat a lot of grass etc. They have access to grit/oyster shell at all time. This weekend I have tried to crush the oyster shell finer, added some crushed eggs shell and a crushed Calcium and vit D tablet to their pellets.
Can I give them too much calcium?
Any other suggestions?
Thanks

Have you tried limestone flour? 
How old is your chuck?
It is not uncommon for chickens to lay soft eggs and it is a classic sign of a lack of calcium.   Are her poos normal?

http://www.chat.allotment-garden.org/index.php?topic=17568.0

Her comb is it as red as the other chucks?  Is her tail down?
Do you leave the feed out all the time?
When a chuck first comes into lay it can cause them problems until the body adjusts.
http://www.omlet.co.uk/shop/shop.php?product_id=128
Quote "Chicken spice is in fact a mineral supplement to keep your chickens in tiptop condition. You might give this to your chicken if it is moulting or to help it maintain a good appetite in winter. You give 1/4 teaspoon of per 2 chickens. Chicken spice also goes by the name of poultry spice."
No it is not normal but it is common is birds coming into lay.  As long as she is laying and she is happy I would not worry to much.   You are doing the right things.  You can not make a chicken eat oyster shell or even egg shell and you can not be sure they are eating it.  But if you mix in to the feed some limestone flour then they will consume some supplement. 
Have a read through this  post
http://chat.allotment-garden.org/index.php?topic=33548.msg403584#msg403584