Probably overcooking it, or adding some flavoring ingredients too soon. I thought my risotto was all wrong because the rice grains were, to my taste, too firm in the middle. Then I went to Slovenia. The part of Slovenia that borders on Italy has food anyone would call Italian. I realized that the grains are supposed to be a little firm in the middle. I was adding more liquid to try to get the entire rice grain soft and turning it into mush.
Cheese, in particular, can make it turn "gloppy" and should only be added at the last minute. too much cheese, especially added too soon, can result in a mess.
Another thing is to have your stock/water/broth HOT and on a back burner. If you add the liquid cold or even room temperature, it doesn't work as well.
By the way, when I first moved here I could not buy the rice meant for risotto. So I learned to make it with Cal-Rose short grain rice, and it turns out fine. But if you're trying with a long-grain rice, the ends of the rice will disentigrate and again, you'll wind up with a starchy mess.