Account always in arrears when paid on time

I have an account with sheet street and pay them every month on time via EFT as it is convenient for me because of my long working hours. However there has been numerous times that after i pay on the end of the month , the 8th of the next month it shows my account is in arrears. I then have to send my proof of payment to the accounts department in order for them to update my account . I think this is getting ridiculous. And i am so worried that they are going to give me a bad name as a bad payer , when in actual fact i pay every month on time. I even asked if they want me to send my Proof of payment to them every month and they asked me not to . This is worrying and cannot go on like this . I want to know what i should do going forward as i cannot go through this every month