Are you using any sugar in your rice? I watched a special on the food network about making sushi rice and they said that a sugar/vinegar mixture seems to stick together better.
Here's a recipe I found on The Food Network. I've never attempted, but let me know if this helps!
5 cups short-grain sushi rice
6 cups water
1/2 cup rice vinegar
2 tablespoons sugar
1 teaspoon salt
Rinse the rice in cold water while stirring briskly to remove any dirt. Drain the rice completely. Place the rice and the 6 cups of water in a medium sized saucepan and cover it with a tight fitting lid. Bring the water to a boil over medium heat. Allow the water to boil for 3 minutes and then reduce the heat to low and continue cooking 15 minutes without removing the lid. Remove the rice from the heat and remove the lid (the water should no longer be visible). Turn the rice out evenly on a well-greased cookie sheet using a spatula or rice paddle. Sprinkle the rice with the vinegar, sugar, and salt while mixing with a spatula or rice paddle until the rice reaches body temperature. Keep the rice covered with damp paper towels or napkin until the rice is ready to use.
Julie and the little darlings...
Tess (fawn girl, born 23-Jun-03)
Conner (fawn boy, rescued 11-Jun-05 @ 8 months)