Questionable security at FNB

"I lost my bank card last month, reported it as lost, and blocked it. However, this month, a transaction occurred on my old card, leaving my account in the negative, and i asked to be refunded the transactions but i was told they will investigate before they can refund me. My question is, how can a transaction still happen on a card that was lost and reported? Does reporting a card as ****** and blocking it not matter? What does this say about their security?"