According to all the paella recipes I've been following you are supposed to use a paella pan and once you add the rice and the stock you are not to stir the rice but let it simmer until it's cooked. The problem I have is that the rice does not cook evenly. The rice in the bottom portion cooks while the rice that is closer to the top remains undercooked. Can anyone tell me how I can ensure the rice cooks more evenly? Thanks.