Poor quality shell

One of my hens is consistently laying large eggs with poor quality shells. She is also very scruffy and has a bald spot on her chest. Any ideas?
Edit: They get layena scratch with oyster shells and plenty of kitchen scraps. Their bedding is pine shavings from the pet store. The other hen lays 'normal' eggs (normal size, normal shell), but only half as often as she did even a month ago. I am going to start turning their light on again to see if that will increase laying frequency, but am wondering if maybe they have mites?