oldmcdonald,
Can you see any little parasites on your hen? Look on her head and legs.
Part the feathers and examine her bare skin.
Watch her and see if she is picking at herself/itching.
How long have you had this hen?
Do you have any other birds?
Any problems with any others?
Any new birds into your flock?
Have you recently changed the bedding in her coop?
Dr. M