Some of the nipples had a softer texture - baby would latch and the entire nipple would collapse and make the air flow hole unusable. I bought 6 bottles from a drug store and added 4 more bottles. I only noticed this issued when I added these 4 bottles to the feeds. I wish nuk had better quality control. Not all 4 bottles had issues, it only happened with 2 of the nipples. Ended up throwing them up since baby was unable to drink milk and would cry from frustration.