Why Is My Shih Zu Puppies (4 Mos) Hair Soft, Fluffy And Thick On Her Head, But Not Her Body?

I have a 4 month old Shih Zu and her head has the same type of coat she had at 6 weeks. It's soft, fluffy and thick, but her body is course. She has had only one professional cut. I give her a bath every two weeks or so, using cleansing wipes and refreshening spray in between baths. The last two baths were with Adams flea and tick shampoo.