I am coming to believe that calcium absorption (as opposed to intake) is a big part of whether a hen can lay.  I had one who laid fine and one who got egg-bound (a calcium uptake deficiency, I've learned) on the same feed.  Different breeds, so different abilities to absorb calcium. 
If your problem hen is dropping soft-shelled eggs, she is a fine layer, but she has a calcium uptake problem.  I would try supplementing her with calcium by dissolving a calcium vitamin tablet in a very diulte solution of vinegar and water, then adding this to their waterer.  How many calcium tablets depends on how many hens you have, since I dont think the calcium can hurt the other hens.  I would try one calcium tab (300 mg) per hen.  Do that for a week and see if the situation improves. If not, maybe there are better ways to get bio-available calcium into this hen.  Just because they're eating layer feed doesnt mean that they can absorb all the nutrients in the feed. I think your hen has a different metabolism and just needs some different management in order to lay for you.