Inaccurate bank charges

Every single month I am getting charged for items paid no funds for one thing or another. Right now I have a R135 charge. This is in spite of the fact that I made the purchase when there is money in my account and money was deducted from it. How is this possible??? I am yet to receive a satisfactory explanation as to why this is. I have even tried speaking to an agent but to no avail. I need answers and to have this charge reversed