There's usually only a few possible causes..
Parasites (external).. check the rest of the skin below the vent and on the abdomen.. is it regular smooth and light pinkish looking or angry red maybe even scabby? Get a look at feather shaft bases while you're there.. any "dry dirt" clumps near the skin? Doing this after dark using a flash light can give the clearest picture as it makes it easy to see things running away from your light. Each bird has their own immune system, forage and bathing or grooming habbits so may not be equally effected or symptomatic. Still if one needs treating, do treat them all.
Motling.. sometimes mini molts.. usually pin feathers can be seen coming in. Did she have a standard molt yet, this past year/season? If not she may be coming due.
Prior or current flock antics, picking.. maybe a fight between those two girls? If it was the back of the neck then roosters would be suspect.. but not usually the front.
Personal over grooming (or overgrooming from a friend), broody plucking (usually farther down ont he belly), or scratching with their own feet.
*Sometimes* old damaged missing or broken feathers may NOT be replaced until they do actually molt.
Those are the top likely suspects.
Other things that can effect feather quality is nutrition. Diminished protein (amino acids) being chief concern.
Hope this helps narrow it down some.
