chicken laid / had a broken egg

we have a girl - goldenline-  (approx 2 years old) who has been laying soft and shellless eggs for over a month now.  Today she looked as if she had a prolapse but there was part of the shell still inside her - we were able to easily remove the remainder of the shell and the prolapse has righted itself.
we have done all the normal things for soft eggs - vitamin d supplement in water, grit and oyster shell in feed but nohing has improved it - other two are laying very hard shelled eggs.
does anyone know what else we can do for her - is she likely to continue to have partial prolapses - as we are not always around in the day to tidy her up and remove any discomfort it may cause.
any suggestions greatfully received

Please dont tell anyone but i did not have any calcium so i ground up a rennie and gave it to her on half a cherry tomato for three days and this sorted her out.

no need for coronation girls to tell anybody at all as we all know now 

I'm intruiged!  What's a rennie?!
Please tell me soon as I'm imagining all kinds of things! 

key ingredients are calcium carbonate and magnesium carbonate.

we have taken our girl to the vets today - as she has had a bit of rough weekend with a couple of prolapses - but the good news is that the vet says not to be overly worried, the prolapses should correct themselves and she should right herself if/when she has a good moult - which we have not had yet even though she is now 2 years old and has laid nearly every day for over 18 months - I just wish she would give herslef a break!