Finding half cooked rice in your rice cooker (or pot if you that is what you use) is a frustratingly simple problem to solve. The problem is usually caused by not adding enough water to the mix.
When cooking rice, the rice absorbs water slowly, until all the water is used up. If there is not enough water, the rice will absorb all the water until it has been all used up. The rice will be undercooked, firm to touch (instead of fluffy). You may even find that the bottom layer has burned, creating a layer of crispy, browned rice.
The rule of thumb is to add about 1.5x to 2x ratio of water to rice. If you cook 2 cups of rice, add 1.5 times more cups of water: equaling 3 cups of water. Or, use the Mt. Fuji technique that you can learn about in this article post: Learn How to Cook Rice.
To wrap it up, make sure you are cooking with enough water. If you are still getting undercooked rice, continue to add more water until your rice is fluffy. Warning: If you add too much water, you may get sticky, pasty rice. Some people enjoy their rice this way, but others do not. So the correct amount of water you use is a matter of opinion. You can read more about the correct amount of water to use on the How much water? article.