Generally speaking, this situation is caused by reasons such as the type of rice and the unbalanced ratio of water to rice.
(1) The type of rice.
Different types of rice absorb water differently.
Short-grain white rice: 1 cup (200 ml) water to 1 cup (200 g) rice.
Long grain white rice: 1 ¼ cups (300 ml) water to 1 cup (185 g) rice.
Brown rice: 2 cups (475 ml) water to 1 cup (190 g) rice.
Brown rice retains the outer layer (peel, seed coat, nucellar protective layer) to protect the tissue so it is not easy to be cooked.
You’ll need to add more water when you cook brown rice, extending the cooking time to get it cooked.
(2) Too much starch.
The grains move around as the rice is transported, causing them to rub against each other and scrape the starch off the rice.
When you start to cook the rice, these starches swell and gelatinize.
As the starch absorbs water, the rice grains begin to be forced closer together, causing them to “stick” together.
I recommend that you wash the rice in cold water before using the rice cooker to cook it.
When washing the rice you will see the water is cloudy, this is the starch shed from the grain.
You can repeat this cleaning process a few times until you get completely starch-free water.
(3) How to fix mushy rice.
a. You can transfer the mushy rice to a pot and heat it on the stove over low heat.
Low heat helps evaporate any excess moisture that isn’t drained.
Once the rice is dry, let it sit for a few minutes.
b. You can put a slice of bread on the mushy rice to let it absorb the excess water.