Why am i not losing weight?

I have been on a diet for a month now and have only dropped a few pounds. I am 5"4' and weigh ten stone. For breakfast I am eating Bran Flakes, an apple mid morning, salad sandwich and apple for dinner and then beans on toast on an evening. I am drinking four pints of water every day and exercising for at least one hour each day. I am not snacking and am trying really hard but the weight doesnt seem to want to come off - any ideas why?