What are you feeding now? Soft eggs are usually due to poor calcium absorption. It can happen even with adequate calcium in the feed. Oyster shell free choice is usually provided even when feeding layer feed since individuals have differing calcium needs.
If you are already providing oyster shell, be sure to check the condition of the shells. If they've been reduced to powder, the particle size is likely too small for it to do much good. Larger shell particles are necessary for adequate calcium absorption since they will remain in the digestive system longer than tiny particles.
If diet is adequate, and the hen still is laying soft eggs, a calcium supplement may need to be given until she's laying normal eggs.