MTN reject additional line on contrcat

I went to MTN to add another line to my contract and they rejected the request. I need to know why please? I have been with MTN for many many many years. My account is not in arrears etc. I want to cancel my Vodacom contract [daughters phone on my name] [it is done now] and move to MTN to pay one account per month. Can MTN help me please to get this done?