Just a question how would like or mites cause soft shelled eggs?
I'm not exactly sure. But the general idea is that lice or mites are very common in chickens and can easily multiply to large numbers. A few lice/mites are not much of an issue, but thousands (or tens of thousands) can cripple or even kill a chicken. If chickens are not laying properly, one thing to always check (assuming they are getting proper nutrition and aren't in a molt) is their health.
I assume you checked their vents. IIRC, the mites are often tiny red colored things, very tiny and you will need good lighting or even a flashlight to see them (and good eyes!). We treated ours with ashes from our fireplace and it worked great.
Worms are another issue that can affect health and egg-laying, but I really don't know anything about them as we don't have an issue with them in our dry climate.
What brand of layer feed are you feeding? Also, check the "milled on" date of your feed. Some feed possibilities could be: poorly stored feed that has degraded in nutrition, expired feed or poor quality feed to begin with. The feed store you buy the feed from should be storing their feed properly (out of direct sun and as cool as possible).
Things I look for in feed as important: some source of animal protein and probiotics (though you can make your own probiotics if you just ferment the feed). Organic is also important to me, but may not be for you. Do keep in mind that if your feed has corn or soy in it (nothing wrong with corn or soy by themselves) and it is not organic or GMO-free, then that corn and soy has been sprayed with MASSIVE amounts of toxic pesticides (aka Round-Up). I don't know if studies have been done on the effects of GMO corn/soy on chickens, but there is plenty of research about it otherwise.
Also, keep in mind that sometimes hens will lay some soft-shelled eggs. Perhaps she's had some sort of digestive upset (feeding some probiotics in their water is a good idea in that cas). But if it has been going on for a while now, then yes, you should do something about it.