we have 5 ex batts and have the odd softie. As well as having it in pots, we mix the grit/oyster shell in with their mash. We've also tried some limestone flour in with their mash but the usual culprits still lay the odd softie.
you can get limestone flour from most horsey places.
also, save the egg shells, bake them in a moderate oven for about 10 mins then crush them and add them back to the food, about a tblsp or so.
lay off the treats for a few days to see if that helps because if they're not filling up on their mash then they're getting enough calcium from their food.
Ii all that fails then I think it's just something they do when they're nearing the end of their laying career as we've tried everything and still get the odd softie.