debit order going off on the wrong date

when i first app**** for a loan with finchoice i asked that my debit order must run on the 20th. That is not always the case. If my salary date falls on the weekend they deem it necessary to debit a day before and then i get unpaid debit orders on my cheque account. This has caused me a great deal of embarrassment and is really damaging my credit profile. I ask that Finchoice at least move my debit order to the 21st monthly . I have called their offices before and requested they change it and was refereed to my contract which says the 20th not 18th or 19th. I'm trying to buy a house and this is really getting in the way because it implies that i dont honour my responsibilities. All i ask is the debit order to be moved on the 21st...Thats all.