The Advanced Anti-Colic Teat's round dome valve, which fits onto the bottom of the venting tube is very sensitive as it is made of specially designed silicone material. This can sometimes stick together when not used for periods of time or before first use. To ensure the slit in the dome valve is open you should wash all parts in warm soapy water, rinse in clean water and sterilise.
Reassemble your bottle as per the instructions. When assembling your bottle it is important that you check the valve at the base of the stem by holding it to the light to check that the slit in the silicone has not sealed closed. If it has sealed closed, pull apart gently between your fingers to re-open. If the valve has sealed closed your bottle will not vent properly.