What are you feeding the hen? A hen needs sufficient levels of calcium, phosphorous, manganese and vitamin D3 in her diet to ensure she has what she needs to produce good quality egg shells. A good quality commercial layer feed will take care of that.
Have you treated the hens for anything, any disease in the flock, or any precautionary treatments? Certain medicines, such as sulfa drugs can cause egg shell problems.
What is the weather like by you? Hot weather, with temperatures over 90*F can cause egg shell issues.
As can certain diseases, such as Newcastle disease, infectious bronchitis, avian influenza.
How old is the hen? There is a higher incidence of egg shell issues in older layers.
The problem can be hereditary. If a hen lays eggs with poor shell quality she can pass it on to her female offspring.
And finally premature laying of the egg. Sometimes a hen may get disturbed during the shell formation process and lay the egg before the process is complete.