Inferior shell quality

Hi I am having problems with the quality of my lemon cuckoo pekins eggs, they are laying eggs with really porous shells they look ok but you can see it when they are candled, this is their second year so they should be at their peak -they are fertile but dont hatch.

They are fed layers pellets, regularly get poultry spice, cod liver oil, and acv and always have oyster shell available, I have four hens and usually three out of four eggs  are like this, any help appreciated.

Did'nt expain  the condition of the shells very well they are not porous like a normal egg but sort of patchy look like are going to be soft but their not, oh been breeding poultry for over 30 years and he doesnt know what it is

Hi no they are not related , will try the extra calcium and vitamin D, thanks for your help