It sounds like she may be having oviduct problems. Unfortunately reproductive, such asimpacted oviduct, salpingitis, egg material coming from vent, shell-less eggs, and others are problems are common in most flocks with chickens over 1-2 years. Since she is 18 months old, are you seeing any evidence of her molting? Is her vent area clear or red and irritated? What does she eat? I would be tempted to give her a human calcium tablet with vitamin D, such as Caltrate or the generic form, for about 3 days. D is important as calcium is to lay a hard shell. But if she appears to be molting, I would wait until that is over, since hens do not commonly lay during a molt. Some will lay shell-less eggs during a molt if they continue to lay.
Viruses, such as infectious bronchitis may cause all kinds of egg shell quality problems. Here is some reading about common reproductive problems:
https://wagwalking.com/bird/condition/impacted-oviduct
http://www.poultrydvm.com/condition/egg-peritonitis
http://www.theveterinaryexpert.com/backyard-poultry/egg-yolk-peritonitis/