Through the power of a vacuum seal, suction cups conveniently hold items, such as decorations, to glass and other smooth surfaces. In order for a suction cup to work properly and not lose suction, you may need to prepare the glass and the suction cup properly before attempting to stick the cup on the glass. This is not difficult to do and will keep the cups stuck firmly for a longer time.
Spray the glass with glass cleaner, and wipe it clean with a soft cloth. Suction cups won't stick well to a dirty surface
Rinse the suction cups with warm water, and add a little soap if they're dirty. If you use soap, rinse them off again and then dry them with a cloth. Just as if the glass is dirty, dirty suction cups will not stick as well.
Dampen your fingertip with water and lightly moisten the inner rim of the suction cup. Moistening helps create a tighter seal. Press the suction cup firmly to the glass and hold it for a few seconds and then gently release the pressure. The suction cup should stick firmly to the glass—give it a very gentle tug to make sure. If it slides at all or comes loose, you may have gotten the rim too moist. Dry it and try again.