Debit order

I have a personal loan and signed a debit order that they must debit my account on the 20th of every month salary day, every time when the 20th is on a weekend the debit order goes of on the last week day of that week e.g the Friday ,my bank returned the debit order ,I took it up with the branch manager and he assured me that he will sort it out and come back to me (still waiting)this month it happen again .can anyone please explain why does happen.and who needs to pay the cost for return debit order.