Interest being charged on account

I am being charged interest monthly on my Edgars account after making an online purchase. Issue was rectified until April 2024. May 2024, onwards I was charged. I phoned in May which the lady said they will adjust the interest on my next statement which did not go through. I phoned again last month and I was promised that the issue would be sorted and yet again I am being charged interest. Why must I constantly phone to get this rectified and my issue doesn't get sorted out