Why is my sushi rice still crunchy?

I don't have the privalege of having a rice steamer, so I boil my rice. I follow the directions to the T. The first two attempts I had with this method came out perfect. I was so happy, but for some reason this time is a disaster. It calls for 1 and a half cups of sushi rice, 2 cups of water. Both poured into a pan and brought to a soft boil, then turned down low to simmer and covered for 20 mins.....As I said, the first two times it came out perfect. This time it is still rather crunchy and I have no idea whiy