Unnecessary interest charged on my Discovery Bank Credit Card!

I am constantly charged interest on my credit card within the 55 day interest free period which is much advertised by most banks including Discovery Bank. I have asked for a response as I am someone who doesn't want to pay interest on my card and therefore pay the full balance just before the 55 days interest free period has elapsed, in order to dodge being charged interest. Despite this, Discovery Bank charges me interest before 55 days (as early as day 30) and cannot and doesn't want to give me a reason why they do so. I am entitled to a full 55 day interest free period, in line with what they advertise to their customers when offering them a credit card.