One of the big focuses that Debenhams pushes is to deliver outstanding customer service at all times, which is something that you do well and the company is right to push this. However, there are some customers who really do just want to be left alone to shop and they will avoid you like the plague. So whenever you approach them and ask if they're alright, you'll get given the "I'm just looking" reply. Well, you weren't asking them what they were looking for, you were asking them how they were. Some people are rude, that's inevitable, but try not to cry on the shop floor when someone explodes at you for asking them if they need any help. Hopefully they were just having a bad day.