I have been using Rick cooker for my whole life. The simple answer I can give you is the problem of your rice cooker.
There are older models of rice cooker that sticks a lot and there is very little you can do about it. It happened to me when I was little that some of the rice cookers just did that regardless of what we did. The newer models won't do it unless inside it's scrated. The Japanese brand rice cookers are very reliable, so you won't have problem like this. I don't recommend you putting oil in your rice as I don't think it's an healthy option.
If there is no obvious scratches inside your rice cooker, then I think it's time to get a better rice cooker.