Egg without shell

Hi everyone.  One of my marans is not laying properly.  The first time she tried to lay she went into meltdown, cold legs, hunched back and very quiet (I posted this on a previous thread).  However she recovered fine but I have become suspicious over the last couple of weeks as I was only getting eggs from the other 3.  On cleaning out the henhouse I have found on a couple of occasions a soft shell egg but today found a shell-less one.



They get layers ad lib, free range most of the day at weekends and 3/4 hours every night when I get home from work.  They have oyster grit available but don't seem to be eating any.  However, there is no problem with the shells on the other three.  They do get treats of corn and at bedtime mealworms or grapes.  Have I not got the diet right?  When she's not trying to lay she's appears happy and healthy scratching around with the others although her tail still seems a little hunched most of the time.

Any advice please?  She is about 28/29 weeks old now. 

Thanks Joy - I'll give it a go