Open the valve at the bottom of the S-bottle by squeezing the short sides of the protruding part together, or by pressing the protruding part in. You will see a small air hole.
Also check that the hole is still open after washing the valve. After washing, the hole in the valve can sometimes get stuck shut. Be sure to check that the air hole in the valve is open before each use. If a vacuum occurs within the S-bottle, this means that the valve at the bottom of the bottle is not working properly. To ensure that the anti-colic S-bottle can function properly, the air hole in the valve needs to be open!
Make sure that the valve is completely dry before placing it into the bottom of the bottle. Press the valve firmly into the base ring all around. Avoid screwing the base on too tightly before using the bottle; simply screw it on with a firm hand.