At 12 weeks the pullet is too young to be molting. She should have finished feathering out at about 6 weeks...
You'll need a strong light source, and if you don't have great vision like me, you might need magnifying glass too.
You have to thoroughly examine the pullet, wrap her in a dish towel to restrain and calm her. Sit in a chair and set her on your lap under a strong light.
If she is not used to you picking her up, stroke her wattles and reassure her with soft voice.
When she is calm, start looking carefully through her feathers and down. Search the neck, back, under wings and the legs.
You are looking for mites and/or lice. The mites will be on the skin and feather and lice prefer to live on the feather as they munch away at the barbs. Also, look at the base of the feathers where these bugs will lay eggs or nits.
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If she is infested, which will cause feather loss, you'll see them. Gross...but you can do it.
Examine the skin while you are at it. Does it look healthy? No patches of irritation, scabs, white or gray discolorations?
Next, check the legs. The scales should be smooth and not lifting or crusty looking.
Mites will get up under the leg scales which is very painful for the chicken.
If she is clean, then, it could be a nutritional deficiency, sometimes even a combination of nutritional and parasitic.
Hopefully you will solve this mystery, until then, feed her some extra protein. A couple of teaspoons of meat protein that you may have in the fridge will do. If you don't have meat available, some freeze dried meal worms will help.