my first chicken was laying for nearly 3months good hard shelled eggs and then one day i went to pick up her egg and my fingers went right threw it "Ewwwwwwww!!! Slimy!" the next day her eggs were right back to normal hard shells. Everyone told me that it was normal for her to have an occasional soft egg, so i dismissed it as being no big deal. But, then two weeks later she laid another soft egg and they stayed soft for over a week. it turned out that she wasn't getting enough green food (grass or other vegetation) and that was causing her to be low on the calcium that makes shells hard. So I had to give her extra green leafy foods and start mixing oyster shell into her meal.
Not saying this happens with every bird especially since my others (i have had 13) never had this problem, but you should be vigilant.
