Financing option cancelled on account

I have been a loyal Vodacom customer and have stayed with the company through a couple of unsavoury experiences. I have 4 contracts with Vodacom, and have recently renewed 3 of them. I was phoned by Orbit Cellular in Wellington to inform me that my contract was eligible for upgrade. Upon enquiring and deciding on a contract option something like a superflex 200 top-up option, the same as two recent contract renewals that went ahead without any problems with CPO iPhone 5S as my phone of choice, they informed me that the financing part that the option required, is no longer available for me. When I enquired as to the reason for this, especially as I did 2 very recent renewals without any hiccups, it was stated that 2 debit orders were sent back on my account. I stated that I was aware of it, but that it was an administrative glitch on Vodacom's side. A while back I went into a Vodacom store when it happened the first time, paid the cash and changed the debit order date to the 7th of each month, to make sure that there would be sufficient funds in the account, as my salary gets paid in from 29 of each month.