What MAY help is offering a dish of oyster shell - keep out free choice. It'll help add calcium to their diet.
How old is the girl you suspect has been laying the off eggs? If the extra calcium doesn't help, she may simply have an egg tract issue - something just not right with the parts/organs of that system.
I have a serama who is almost 5 yrs old, and after laying two perfect eggs in a row back when she was around 20 weeks old, she has laid nothing but "jello" eggs since - no shell - just the membrane with white/yolk inside. Just like with humans, or other animals, sometimes organs aren't right due to congenital issue or possible disease.
I've notice that two of my older gals' (6-7 yr. olds) egg shells are getting really thin - I think for them the egg tract is just breaking down a bit and not working as well due to aging.
There is a bit of risk involved with the soft shelled egg laying - it's harder for them to expel the egg. I've seen hens hunched up, acting sick, trying to work a soft shelled egg out - maybe because there's not something solid to push along??? And I'd think there'd be at least a small risk of, if an egg broke inside and not all was passed, that infection could occur. But typically they will get it all out...