soft eggs

i had four chickens in my back yard and i brought two newones over a week ago,they are all around the 20 weeks old and the four that i had to start with,were all laying on average an egg a day,now since i introducind the two new ones,two of my origanl hens have stopted laying and have layed soft eggs,which fall apart when you touch them,they roam around my garden freely and seem happy,but seems im new to this,im just wondering if im doing anything wronge,all though i new adding two new hens would upset them i didnt exspect them to stop laying,or laying soft eggs. help ?

Do they have oyster shell?
Commerical food contains more than enough calcium, but when free ranging, they are likely to get 75% of their diet from other sources.