A blockage in the anti-colic air vent usually happens due to the washing and/or drying process. To prevent the air vents from getting stuck, do gently massage the vents with undiluted baby bottle detergent during washing in order to remove any milk residue. Alternatively, you may also try massaging them immediately after washing, prior to sterilisation, so that water trapped in the vent would not cause the vent to be stuck together upon drying.
Subsequently, we would encourage also a quick massage on the anti-colic air vent to ensure that it is not stuck together before assembling it to the collar for feeds.
Check out our video tutorial here for a step-by-step visual guide: