If you have Phillips Avent natural response they are on drip and really hard to suck from. At newborn ours were using a size 3 flow teat. They quickly moved to a 4 flow around 2 months old (if normal formula/breast milk this is what we still use they are 4 months old) and are now on a 5/6 because they are reflux formula which is thickened so they need a higher flow Just thought I would mention as this may fix the issue you’re experiencing as the slower flows were too hard for my girls to get milk from so they got frustrated and continued to be hungry Edit to add: we exclusively bottle feed due to latching issues as twins were born early and triple feeding 2 babes became overstimulating really quickly. We also use MAM easy start size 0 or 1 when out and about, they don’t do as well with them but well enough and they easier to sterilise when out because they can be microwaved so a great back up

I tried so many different bottles and the only thing that ended up working was offering a bottle of warm formula instead of expressed breast milk!