FNB *****ing my money

Pathetic service! My bank balance for my FNB homeloan is not adding up and FNB is doing nothing to get this sorted out or rectified. I informed FNB 2 weeks ago, lodged complaints and there is still almost R4000 missing from my account. I check my bank balance every month to ensure the debits and credits are adding up but it seems FNB does not know how to do basic accounting. How are you not picking this mistake up and worse, not rectifying the mistake? How much money are you taking from other accounts where people dont check their balances regularly???