What Causes Uterus Bleeding While On Norplant?

im using norplant jadelle and for the past 4 weeks i have been experiencing uterus bleeding. i went to see a gynaecologist and he prescribed for me Norethesterone which i have taken for 7 days and the bleeding stopped. iys been three days now and the bleeding has started again. wat could be th problem and how can it be resolvd?