FNB Refuses to Release Deposit Blocked for Car Hire

I hired a car from Budget Car Hire 8 March. "Avis Rent a Car" blocked R2569.68 on my FNB Cheque Account. I returned the car on 13 March. The money wasn't released within the stated 5 - 7 working days, so I called FNB. They told me they would send me a form to fill out and send back, which I did. No response. Only on the third time did I get a response from someone asking for a reference number. For what?! I am an FNB client. All of the information is on the form. The money is clearly sitting in my account under "Pending". I simply do not understand why they just can't release it. I have called can emailed countless times. I also submitted a complaint through their website and was told someone would get back to me in 24 - 48, which obviously didn't happen either. I do not understand why this is so difficult. Just release the money that was blocked.