I feel your pain. I manage to keep my weight down with the 5:2 diet and plenty of exercise. But I always used to have a flat stomach and recently.....I haven't. My waist's still there, but I'm very conscious of the fat gathering below.
Still trying! I weigh a lot more than I did even a few years ago and I keep trying to eat less/more healthily (not helped by DH who always cooks enough for the whole village) and to get more exercise. The weight creeps down a pound or two but never as far as I'd like.