FNB to FNB payment delay causing FNB to freeze my account? ********!!!

Due to the delay it took for a payment made from my FNB business account to reflect to another FNB personal account, it resulted to my client laying a complaint that I didn't pay them; therefore FNB decided to place a hold on my account. I find it very unprofessional for FNB making me suffer for its own mistakes and delays. I tried calling them to clear the money and they kept saying it might take 12 hours to reflect on the other side. Now they decided to place a hold on my account because one of my clients called them and complained. Please fix this issue immediately because there's no excuse as to why FNB to FNB should take longer than 12hours to reflect; and unplace the hold you placed on my account.