Freezed my account

I have an overdraft with the bank. Last month i visited FNB Wonderpark branch to arrange the payment method, then we agreed that every month i will pay R650.00, then they told me that i must make sure that there are funds available by the first of each month. But because i dont have exact pay day, i told them that I only get the funds in my account between the 30th and the 7th of each month, and this month on the first there were no funds in my account but i only received the funds on the fifth of this month(April), and after i received funds they freeze my account, when i asked why they freeze my account they told me there were no funds on the first of the month, even thou i told them that i only receive the funds between the 30th and 7th. Today i went to Rosslyn branch but never got any help.