If you only find one soft shelled egg, do not worry. There is nothing wrong. If the egg shells are fine except for this one case, they do not need more calcium.
Occasionally a hen's internal egg laying factory messes up and starts a second yolk through the process soon after another one has started. The first egg is normal but it has used up the egg shell material in the shell gland. The hen's shell gland does not have enough time to collect enough shell material for the second egg so it is often a soft shelled or shell-less egg. There are a few other things that might cause a rare soft shelled egg, but her laying one after the other is the usual cause if it is a rare occurrence.
If it is one hen consistently doing his and all the other hens lay eggs with good shells, it is something internally wrong with that one hen. Her body does not process the calcium correctly.
If it is one just starting to lay, it sometimes takes a while for the pullet's internal egg laying factory to work out the bugs. You can get all sorts of weird eggs until they get the internal egg laying factory working properly.
If many eggs are consistently soft, they probably need more calcium or they have a disease. This does not sound like it is your case.
Layer feed has all the calcium in it that they need for egg shells if Layer is all they eat. If you feed them a whole lot of other stuff, they may not get enough calcim so offering oyster shell is a good idea. It actually never hurts to offer oyster shell, but if they don't touch it, they are getting enough calcium from other sources.
I feed Starter or Grower to my entire flock when I have young chickens with the flock and offer oyster shell on the side. If I keep them locked in the run, they eat some oyster shell, but not really a lot. I do feed them a lot of green stuff from the garden. You'd be surprised how much calcium is in some of those plants. When I let them free range, whether I am feeding Layer, Starter, of Grower, they hardly ever touch the oyster shell and the egg shells are fine. They get all the calcium they need from what they are finding to eat, either from the Layer or the bugs, plants, and other stuff they eat.
To sum up. If it is a very rare occurrence or a pullet just starting to lay, you do not have a problem. If it is one hen consistently doing this, your problem is one hen, not the entire flock. If it is flock wide, then you do have a problem.
Good luck!!!