I paid 3 days later now I am a month behind, how does it work?

I changed a debit order on my account from the 28th to the 1 now MFC tell me that I am a month behind, if a payment go through 3 days later how can it be a month late. Now I want to put it back in the same month now MFC is rejecting it. This is unprofessional and unacceptable. Even more that they have high call volumes, and is a struggle to talk to them.