Brittle and soft eggs

We have three hens all cross breeds. They are all just over a year old and have laid regularly.  Maud, who is bottom of the pecking order (my husband likes to think she is the 'artistic' and spiritual one), started to lay odd eggs a couple of months ago. She has always laid the odd soft egg but this has been different.  The shells are less robust than the others but firm at one end but at the sharper end, the last bit out as it were,  the shell is not so much brittle but crumbley- it caves in at the slightest touch. The firm bit is thinner and more brittle than it should be but the egg inside is fine.  They have layers pellets, grass, weeds, (expensive perennials now and then) and all manner of calcium and mineral supplements, grit a bit of corn now and then and a bit of porrige for which they would sell their little chicken souls. Maud looks healthy,  is happy with a good appetite and more at ease with her social status than I am. Does anyone have any idea what is going on? I am worried that sooner or later one of these eggs will break before she lays it.

I've got this problem with some of my ex-batts.  I found it improved when I started adding limestone flour to their afternoon mash&corn ration, though it hasn't stopped it completely.

I've had a couple of problems with Wilma when a thin shell has broken before it's out, luckily have noticed it, and managed to pinch a bit of protruding shell and ease it out for her.  Since LSF, she hasn't had a problem.

We still get the odd soft shell from Paxo though.