problem with my direct debit

Hi I changed my direct debit form my app well a head before due date. You took the money from my other bank account but also refunded the money same day. I didn’t notice what’s going on. 
I open my app few days back and noticed you are saying £10 payment is due. So I make payment straight away. But I’m still confused why you didn’t accept my new direct debit request as you took the money and refunded back. Now I’m receiving letter from you. Can you plz answer me or call me now