Hello i have had the exact same problem with my DS whenever it was teat change time he would cough splutter and choke so we went back to the old ones. Just keep going with the old ones i found that i tried again once a month and then all of a sudden he just took to the new teat when he was ready and could handle the flow. Try not to listen to the bossy packs telling u where ur little one should be up to they will only prove it wrong hehe.