Sustained Absence of Explanations for Interest Charges

I have tried for months to resolve an enquiry regarding interest I am being charged. Even after paying early for months in succession, I am charged interest.

I have three of four call references: (18/3) Man promised to call me back before close of business that day. He did not call back but recorded on the bank's system that he had. I have no message from D Bank nor missed calls from the bank.

(27/3) After again setting out detailed explanation of problem, Goitse promises to call back before COB. She does not.

(3/4) Misho asks that I re-explain. There is insufficient detail on her system for her to pick up where I last left off with D Bank. She puts me on hold. At my request she assures me she will call me back if I get cut off. I get cut off. She does not call back.

Further, D Bank when they charge interest do not make the interest calculation available on their app until +/- 30 days after the debit. If they can make the debit, why can they not explain the calculation thereof?

I don't understand why I cannot get a satisfactory explanation of why I am charged interest when I a) make no cash withdrawals and b) frequently pay early.