Unable to upgrade account

My cellphone contract is due for an upgrade, and when I tried to upgrade to get the HTC One M9 the consultant told me that my portfolio is too low, so I can't upgrade. She then sent a request to the acquisitions department to increase my portfolio which was rejected. A couple days later I contacted an agent through the online portal, which told me that I qualify to get the device I wanted, but when I went to the shop to upgrade, I got the same feedback from the consultant that my portfolio is too low so I can't upgrade. She then requested another portfolio update, which she told me should be done after about 4 hours. That evening I went to another vodacom shop to do the upgrade, but then the consultant told me that the request was rejected again and it is because of a returned debit order a couple of months ago, which I paid within days after it was returned. So why am I still penalised because one month's payment was a couple of days later than normal? My account isn't behind or no payments have been missed, but now I am unable to upgrade my contract and have to wait months before I can upgrade. This is unacceptable and I'm going to cancel my contract as soon as it expires