I agree that calcium/Vitamin D should be the first option, but don't doubt my veracity. I use a quality, fresh feed- Nutrena Hearty Hen and later Nutrena All Flock and have had good success with my girls for 7 years before running into a soft shell problem. Vitamin E was an excellent solution...
I wouldn't do both of them at once. It is best to go with the minimum solution possible. Select one and test it. If it works, you won't need the other.
I'll support the others as a first line solution. If it doesn't work, please ping me. I had a similar problem, but my girl reacted immediately to Vitamin E supplementation.