Unhappy Customer:Claim of unpaid debit order

I had a very bad experience with Full House. In the year of May 2020 I went and purchased a stove. I made a debit order for the 1st of June, now after almost 2 years they claim I didn't pay for the month of May 2020. I want to know who is at fault if my account was made on a debit order, they didn't state initially that I had to pay for the month of May 2020 or twice. I was aware that the 1st payment for May was June 2020 because that was the arrangement. Now they keep on phoning me claiming I am a month behind. They don't even sent statement letters to inform me about my account. Please advise what is a perso n to do pay something which they only noted now or ignore. I feel it's very unreasonable and wrong because of the debit order. Also I went to the store to ask but no 1 made me aware of this.