Soaking the lentils well for at least 4 hours is very important. Under soaked lentils won’t yield a fluffy batter and the vadas turn out dense and hard. Not using enough water while grinding and not aerating the batter enough can also make hard vada. Lastly aged lentils that have been in the shelf for a long time do not make fluffy batter so use lentils that are fresh. Avoid using lentils that have picked up an yellow shade.