Item paid no funds query

Item paid no funds: every single month I make payment as per my current money available to me and then after 2-3n weeks I always have to pay fnb with reason no funds .I have logged numerous queries with FNB with no luck, this still continues every month. I am well aware of pending versus paid transaction .I have been banking with FNB for over 15yrs + and this has never happened before only this year every month it's a issue.