Newborn screening?

My son had his newborn screening done in the hospital. The results came back inconclusive or that something wasn’t right. We went to the health department and had them re done and yet again the same result. We already know he is a carrier of the sickle cell trait, but what could cause it to come back inconclusive twice ? I don’t have much info on it and they haven’t told me much. Just a little worried !