If you added too much water many grains explode and make the kernels stick together. To prevent this from happening add a tad less water. An addition of app. 1 g gypsum (CaSO
4) per pint jar can also help preventing the grains stick together.
Another point is the correct amount of grain used per jar. Once pressure cooked, the jar shouldn´t be more than 2/3 full to allow for easy shaking.