It means that your oats-to-liquid ratio is wrong or you didn't cook the recipe long enough. Keep in mind that the liquid won't absorb at once over the stove.  You must cover the cooked oatmeal with a lid and set it aside for a few minutes for the remaining liquid to get absorbed. However, if this doesn't help, it means you need more oats. You can simply add more oats, one tablespoon at a time, and keep cooking until it thickens up.