Baby bottle teats collapse when you screw the top on the bottle too tightly. The top should be tight enough to not let milk spill on the sides and not too tight that no air can’t pass. The nipple also collapses when the baby sucks up too hard. It could be because you have changed the size of the nipple, or rather it is time to go up in size. Your baby tries to suck his feed to get more, and the nipple collapses. It is a sign you get a bigger size nipple. Eventually, when you get one size bigger, your little one may choke the bottle. They suck hard as they need to with a newborn nipple, not knowing they don’t have to any longer. However, it won’t take them too long to figure out the change and adjust accordingly.