If you've only been having your humidity at 60% during her shed times then thats probably why she keeps giving incomplete sheds. I believe a ball python's default humidity is 50% so 60% isin't enough to shed properly.
If you could bump that up to 75 -80% she should shed completely, or get a tupperware box with a warm damp cloth and keep her in it for a few hours a day while shes in blue.Or maybe even add some sphangum moss to her enclosure.
The primary reason a snake has problems shedding is usually cause the proper humidity isn't being provided,which sounds like the pet store you purchased her from also had the same issue with improper humidity.