It probably has, just not when you are looking. Unless your mare is off colour or not herself I, personally, I wouldn't worry overmuch.
Are you just watching for movement, or resting your hand on the bump? My mare is about 8 months now and I feel the foal moving most when I put my hand underneath her belly just infront of her backlegs.
Foal movement - mmmm! I foal 12-14 mares a year and I rarely see or feel any movement until a couple of weeks - or days - before foaling. Foalie is VERY well padded in there and there's a lot of flesh, muscle, and skin between the uterus and the world outside, not to mention a couple of gallons of fluid.
What people think they see or feel is usually muscle contractions or wind.