This is our first year raising chickens and we have one of our four girls who lays one to three eggs a day and they're all soft shell. Sometimes she'll lay two softies in one sitting. They're 6 months old and have been laying for almost a month. All of them are on layer pellets and have crushed oyster shell mixed in. The other three have been laying just fine with no soft shells. Should i be concerned? Any advice would be much appreciated.