4th leaking bottle

Yesterday, was my 4th nuk bottle that I purchased this year. I bought 3 bottles from Dischem & within weeks, the bottle leaks by the part that says \air system". I decided to buy a new bottle from Checkers yesterday & to my surprise it leaks again. <br> My older daughter is on an airvent bottle & she use this without any issues. I can't remember when last we changed the dummy. Earlier this year"