FNB not returning my money

A transaction went off my account in early November at a local grocer(one of the biggest in the country) but the POS didn't confirm the sale. The grocery store manager was very helpful and even gave me a letter from their Head Office to confirm that they indeed had not banked the funds- in this letter, they plead with FNB for them to assist me in retrieving these funds.

I sent my bank statement together with this letter from the chain store to FNB and a month later, nobody has even bothered to call me. I've tried everything from secure chat on the App, to visiting three different branches, to calling almost every 087 FNB number.

This is quite a bit of money- in the thousands.