Suction cups stick better when the surface to which they are stuck and the suction cups themselves are clean and free from dirt. If necessary, wash the suction cups with warm, soapy water and pat them dry. Then, clean the surface before sticking the suction cups onto it.
Push the suction cup against the surface until all the air bubbles inside are released and a vacuum is created, which firmly seals the cup to the surface. Adding a little oil or petroleum jelly to the rim of the suction cup can help keep the seal tight. Also, the surface must be smooth and non-porous, such as glass, tile or metal, to ensure good suction.