Spotting after miscarriage could be due to either organic or functional reasons. Organic reasons for vaginal spotting can include the presence of either an infection, a tumor, endometrial hyperplasia, placental remains after delivery or miscarriage or could also indicate injury. It seems that you underwent the medical investigations to confirm or exclude organic reason for the spotting. Spotting can be also due to some kind of hormonal imbalance, which is called "functional bleeding". In cases of funcational bleeding, progesterone is usually used to treat the symptoms.

In cases like yours wherein preliminary medical tests excluded an organic reason for spotting and hormones didn’t help treat the supposed functional bleeding, the only diagnostic tool that remains is endometrial curettage. Furthermore, endometrial curettage is not only diagnostic tool but also used as a treatment tool. You can consult your gynecologist about this to request more information and to further seek answers that point to a diagnosis.