Soft shells are often seen with young layers, but if the hen has been laying for awhile it could be caused by nutritional deficiencies (though it sounds like your hen is getting more than enough of what she needs), internal or external parasites, weather extremes, certain diseases, etc. More info on that here:
 
https://www.backyardchickens.com/a/common-egg-quality-problems
 
Be careful not to feed 
too much calcium rich food. Too much calcium in the hen's diet can also cause egg shell quality problems. If she's getting layer feed, which is specially formulated to meet layers' nutrient requirements, and you offer oystershell or eggshell free choice it will be enough.