If your oil stick doesn't leave behind a nice smooth oil paint trail, it may have a skin or film preventing it from doing so. Try removing a thin layer from the tip using a paper towel or the edge of a palette knife. This oxidized skin naturally forms, and ensures you can use the remainder of your oil stick. If you prefer to slow the formation of this skin, you can wrap the sticks with plastic wrap and store the sticks in as airtight a container as you can procure.