UNPAID ORDERS THAT WERE DEBITED

I took an investment with FNB last which they were debiting R500 on monthly basis. Everything was running smoothly until this year in October whereby a debit order went through and later came back as an unpaid item. I called to find out as to what is happening but there was no answer to my question, I then visited the branch whereby they told me they were not sure as to what happened but the money was reversed back to my account. Upon checking my statement there was never any reversal of that amount.

The same has happened again this month, R530 was debited on my account and later came back as a unpaid. I called again to find out as to what might be the problem and there was never any actual reason, as a result I wrote an email to the bank and raised my concern and di-invested my account as there was no one willing to explain where did the money go. I later received an email stating that it was a system error , which I believe that after noticing their mistake they should've refunded me but they've never done so.