Failed collections on 2nd contract account

Hi Experts,

A business partner has two contract account A an B. The direct debit works for contract account A but it doesn't work for contract account B. When i see the returns lot in FP09 tcode, i find that the returns lot A has an particular item  which has StR F5U . The reason of reversal given for F5U having Housebank POST is "invalid transaction code".

What could be the possible reason why direct debit work for a contract account A but not for contract account B of same Business Partner.

Thanks and Regards

Dushyant