Idlis come out flat if the proportion of Urad dal to rice is not balanced. The ratio will differ when grinding in a wet grinder (traditional method) as opposed to grinding it in Mixie (processor). For wet grinder I tend to use 3:1 (rice:urad) ratio. I haven't tried mixie yet hence have no recommendation. Some readers of mine suggest reducing the amount of rice in that case. Try 2:1 to see if that proportion works. Too much water is also responsible for flatter idlis. The consistency of the batter should be smooth, velvety, kind of foamy - like a ketchup.Not thick and not watery either.