I feel like this is a really dumb question, but how do I know when DD is having to suck so hard on the bottle that the nipple collapses, and when do I need to bump up to the medium flow as a result? We use the Nuk bottles with the airvent system and they've worked well up until a couple of days ago. Her mouth will move a few times like she's trying to suck but she's not swallowing. She'll then stop and unlatch, or I unlatch her myself. When she does that, bubbles come up from the nipple and she can eat again. This doesn't happen all the time but it is happening more frequently. We get through a feeding (3-4 oz) pretty quickly, about 20 minutes including burp time so it's not like she's taking a super long time to eat and doesn't act frustrated yet so I don't *think* it's time to go to a faster flow. I've checked the airvent hole in the nipple base and it's not clogged.