liquid calcium

Hello.
One of my ex-bats has laid soft eggs for the last 7 weeks.
(they are on ex bats pellets with oyster shell and they all eat the same yet she is the only one with this problem...)

I am now administering 2ml of liquid calcium/vitamin D every day and have done so for 10 days.

So far no improvement and still soft eggs a few times each week (no hard shelled ones at all).

How long should I continue with the liquid calcium?
What else can I do if this doesn't work?

Hi...
Thanks for the advice - I'm just so worried she could get egg peritonitus if she continues with the softies and some gets stuck inside so am keen to get her sorted asap - am also aware I sholdnt be giving the calcium too lon as it's not good for them.
She definitely has off days and isnt as perky as she once was.

She has laid a couple of hard shelled eggs but the tops were soft. Since then they have become softer and softer intil it's just a membrane now...the other day I found her walking around with the egg albumen training out of her and I had to pull the rest out myself!!