poor quality eggs

I 'm wondering if any other member has had a chuck with the same problem as our Treacle(copper star)
She is 2years old or so and lays every day,she is a robust little chook although she is looking a bit tatty due to what i think is a partial moult? her eggs are huge(86g up to 95g)on a regular basis,just lately she has been producing softer,thinner shelled eggs which have a poor quality very watery white that disintegrates when poaching.
Non of the other hens have produced anything like these eggs.
They have all been wormed with flubenvet and deloused with frontline last month
Any ideas please 

hi
we have the same problem.we got yesterday and today soft egg
i believe its because of they dont have enough calcium.

Thanks for the replies,I think they are getting a good intake of dark greens on a regular basis(almost daily) and their mixed corn + seed has added oystershell.
They also freerange a few times a week.

I found them eating a soft shelled egg this morning so now I have two problems to sort out

Hi,yes forgot to mention they are also fed layers pellets and  have tonic in their water
The hen in question laying the softshells has been looking a bit tatty  and I don't know if its of any significance but she also has had a dirty bum for a week or so too. She seems to be her usual chatty self otherwise.

I will try adding extra limestone powder or some crushed eggshells

I had a chicken that laid jelly eggs alot, never got to the bottom of what it was. The other girls were fine and laid really hard shells and they were fed the same food. I think she had really sharp eyes and was a fussy eater so she used to pick out her favourite bits (sunflower seeds)  Maybe she ate so many sunflower seeds that she didnt get enough grit or greens...?

If she has been laying well for her two years, as a hybrid, she may be coming to the ernd of her egg laying career hence the soft eggs. It is worth trying the limestone flour etc. If you are going to use crushed egg shells do bake them in the oven to ensure they are clean first.